Medical Billing & Coding Glossary: 110+ Terms and Acronyms Explained

Medical billing runs on acronyms. Walk into almost any conversation about claims, denials, or revenue cycle work and you’ll hear CPT, ERA, MAC, CO-45, and a dozen more thrown around as if everyone already knows what they mean. If you’re learning this field — whether you’re studying for a certification in the United States, training remotely from Pakistan or the Philippines, or simply trying to understand a bill you received — that shorthand can be the biggest barrier to actually understanding the work.

This glossary explains the terms and acronyms that come up most often in medical billing, coding, and revenue cycle management, in plain language that doesn’t assume you already work in a U.S. clinic. Where a term is specific to the U.S. healthcare system (Medicare, Medicaid, HIPAA, and similar), we say so directly, since understanding how the U.S. system works is often the whole point of learning these terms in the first place — wherever you’re doing that learning from.

A

ABN (Advance Beneficiary Notice of Noncoverage) — A written notice a U.S. provider gives a Medicare patient before a service, when the provider believes Medicare may not pay for it. It lets the patient decide whether to proceed and accept financial responsibility.

Adjudication — The process an insurance company (payer) goes through to review a submitted claim and decide how much, if anything, to pay.

Adjustment — A change to the amount billed on an account, most often the difference between what a provider charges and what a payer’s contract allows — not something the patient owes.

Aging / AR Aging — A report that groups unpaid claims or patient balances by how long they’ve been outstanding (for example, 0–30, 31–60, 61–90, 90+ days), used to spot slow-paying claims before they become uncollectible.

Allowed Amount — The maximum amount a payer will pay for a given service under its contract with the provider, regardless of what the provider originally billed.

AOB (Assignment of Benefits) — A patient’s authorization allowing their insurance payment to be sent directly to the provider instead of to the patient.

Appeal — A formal request asking a payer to reconsider a denied or underpaid claim, usually with additional documentation supporting why the service should be covered.

APC (Ambulatory Payment Classification) — The payment-grouping system Medicare uses for outpatient hospital services, similar in concept to how DRGs work for inpatient stays.

AR (Accounts Receivable) — Money owed to a provider for services already delivered but not yet collected, whether from payers or patients.

Authorization / Prior Authorization — Approval a payer requires before certain services happen, confirming in advance that the service will be covered.

B

BAA (Business Associate Agreement) — A HIPAA-required contract between a healthcare provider and any outside vendor (like a billing company) that will handle patient health information on the provider’s behalf.

Balance Billing — Billing a patient for the difference between what a provider charged and what the payer’s allowed amount was. Increasingly restricted for in-network and emergency care in the U.S. under surprise-billing rules.

Beneficiary — The person entitled to receive benefits under a health insurance plan — commonly used specifically for Medicare or Medicaid enrollees.

Bundling — Grouping several related procedures under a single code and payment rather than billing each one separately, because payer rules consider them part of the same overall service.

C

Capitation — A payment model where a provider is paid a fixed amount per enrolled patient per period, regardless of how many services that patient actually uses.

CARC (Claim Adjustment Reason Code) — A standardized code on a remittance advice explaining why a claim line was paid differently than billed (reduced, denied, or adjusted), such as “CO-45” for a contractual write-off.

Charge Capture — The process of accurately recording every billable service a patient received so nothing goes unbilled.

Clean Claim — A claim submitted with no errors or missing information, able to be processed without follow-up — the goal of an efficient billing process.

Clearinghouse — A third-party service that sits between providers and payers, checking claims for errors and translating them into the correct electronic format before forwarding them.

CMS (Centers for Medicare & Medicaid Services) — The U.S. federal agency that runs Medicare and Medicaid and sets many of the coding and billing rules the rest of the industry follows, even outside those two programs.

CMS-1500 — The standard paper (and electronic-equivalent) claim form used for professional and outpatient services in the U.S. — the counterpart to the UB-04 used by facilities.

CO (Contractual Obligation) — A claim adjustment group code indicating the write-off is required by the provider’s contract with the payer, not something the patient can be billed for.

COB (Coordination of Benefits) — The rules determining payment order when a patient is covered by more than one insurance plan at the same time.

Coinsurance — A percentage of the allowed amount the patient is responsible for after meeting their deductible (for example, 20% coinsurance on a $100 allowed service means the patient owes $20).

Copay / Copayment — A fixed dollar amount a patient pays for a visit or service, set by their insurance plan regardless of the total cost of care.

CPT (Current Procedural Terminology) — A set of five-digit codes, maintained by the American Medical Association, describing the medical, surgical, and diagnostic services a provider performed. Along with ICD-10-CM and HCPCS, this is one of the three coding systems every claim depends on.

Credentialing — The process of verifying a provider’s qualifications (education, licensure, work history) so they can join a payer’s network and bill that payer for services.

Crossover Claim — A claim that Medicare automatically forwards to a patient’s secondary insurance after processing, so the provider doesn’t have to submit it a second time.

D

Days in AR — The average number of days it takes a practice to collect payment after a service is rendered. Lower is generally healthier; well-run practices often target under 30–40 days.

Deductible — The amount a patient must pay out of pocket before their insurance starts sharing the cost of care.

Denial — A payer’s decision to refuse payment on a submitted claim after review, as opposed to a rejection, which happens before the claim is even processed.

DME (Durable Medical Equipment) — Reusable medical equipment like wheelchairs, oxygen equipment, or hospital beds, billed under its own HCPCS codes and, in the U.S., subject to its own supplier and prior-authorization rules.

DRG (Diagnosis-Related Group) — The classification system Medicare uses to pay hospitals a single, predetermined amount for an inpatient stay based on the patient’s diagnosis, rather than paying for each individual service.

Dx — Shorthand for “diagnosis,” commonly used when referring to the diagnosis code(s) attached to a claim.

E

EDI (Electronic Data Interchange) — The standardized electronic format used to exchange claims, remittances, and eligibility information between providers, clearinghouses, and payers — the plumbing behind nearly every electronic billing transaction.

EFT (Electronic Funds Transfer) — Direct electronic deposit of a payer’s payment into a provider’s bank account, replacing paper checks.

Eligibility Verification — Confirming a patient’s insurance coverage and benefits are active before providing a service, to avoid delivering care that turns out not to be covered.

E/M Codes (Evaluation and Management) — The CPT code range covering physician office visits, consultations, and similar cognitive (non-procedural) services — among the most frequently billed codes in outpatient medicine.

EOB (Explanation of Benefits) — A statement a payer sends (often to the patient, sometimes to the provider) explaining how a claim was processed: what was billed, what was allowed, what the payer paid, and what the patient owes.

EPSDT (Early and Periodic Screening, Diagnostic, and Treatment) — A federally required Medicaid benefit in the U.S. covering preventive health screening for children and young adults.

ERA (Electronic Remittance Advice) — The electronic version of an EOB, transmitted directly to the provider’s billing system so payments can be posted automatically instead of by hand.

F

Fee Schedule — A complete list of the maximum amount a payer will pay for each billable service.

Fee-for-Service — A payment model where a provider is paid separately for each individual service delivered, as opposed to capitation or value-based models.

Formulary — The list of prescription drugs a health plan covers, and at what cost tier.

Fraud and Abuse — Billing practices that are dishonest (fraud, such as billing for services never rendered) or simply inconsistent with accepted medical and billing standards (abuse, such as routine upcoding), both of which carry serious legal and financial consequences.

G

Global Period / Global Surgery Package — A Medicare policy bundling a surgery’s pre-operative, the procedure itself, and routine post-operative care into a single payment, so related follow-up visits within the global period usually aren’t billed separately.

Guarantor — The person financially responsible for an account, who is not always the patient (for example, a parent is usually the guarantor for a child’s account).

H

HCPCS (Healthcare Common Procedure Coding System) — A coding system that extends beyond CPT to cover items CPT doesn’t, such as ambulance services, durable medical equipment, and injectable drugs (its “Level II” J-codes).

HIPAA (Health Insurance Portability and Accountability Act) — The 1996 U.S. federal law that set national standards for protecting patient health information and for the electronic transactions used in billing.

HMO (Health Maintenance Organization) — A type of health plan that generally requires patients to choose a primary care provider and get referrals before seeing specialists, in exchange for lower costs.

I

ICD-10-CM — The diagnosis coding system used to describe why a patient needed care — the “why” that pairs with CPT’s “what was done.” See our full ICD-10-CM Coding Guide for how these codes are structured and the FY2027 update.

In-Network — A provider who has a contract with a specific insurance plan, agreeing to accept negotiated rates in exchange for being listed as a covered option for that plan’s members.

Incident-To Billing — A Medicare billing provision that lets services performed by a non-physician practitioner be billed under a supervising physician’s NPI at the physician’s full fee schedule rate, under specific supervision conditions.

Inpatient / Outpatient — Inpatient refers to care requiring formal admission to a hospital; outpatient refers to care that doesn’t, even if it happens at a hospital-owned facility. The distinction changes which coding and payment rules apply.

J

J-Codes — A subset of HCPCS Level II codes (J0000–J9999) used specifically for injectable and infused drugs.

K

KX Modifier — A modifier required on certain Medicare therapy claims once a patient exceeds a set therapy-cost threshold, certifying that continued treatment is medically necessary.

L

LCD (Local Coverage Determination) — A coverage policy issued by a specific Medicare Administrative Contractor, applying only within that contractor’s geographic jurisdiction — as opposed to a National Coverage Determination, which applies everywhere.

M

MAC (Medicare Administrative Contractor) — A private company contracted by CMS to process Medicare claims for a specific U.S. region.

Medicaid — A joint U.S. federal and state program providing health coverage for eligible low-income individuals and families; rules and coverage vary by state.

Medical Necessity — The standard payers use to decide whether a service was appropriate and required for the patient’s diagnosis or treatment — the most common basis cited in claim denials.

Medicare — The U.S. federal health insurance program primarily for people age 65 and older and certain people with disabilities.

Medigap — Supplemental insurance policies that help cover the costs Medicare itself doesn’t (like coinsurance and deductibles).

Modifier — A two-character code appended to a CPT or HCPCS code that changes or clarifies how a service should be interpreted — for example, indicating a procedure was performed on a different side of the body than a related, recent procedure.

MSP (Medicare Secondary Payer) — The set of rules determining when Medicare pays second, after another insurance plan (such as an employer plan), rather than first.

MUE (Medically Unlikely Edit) — A CMS-published limit on the number of units of a given code that could plausibly be billed for one patient on one day, used to catch billing errors automatically.

N

NCCI (National Correct Coding Initiative) — A set of CMS coding edits designed to prevent improper payment when code combinations shouldn’t be billed together.

NCD (National Coverage Determination) — A nationwide Medicare coverage policy, issued directly by CMS rather than by an individual MAC.

Net Collection Rate — The percentage of the money a practice was actually entitled to collect (after contractual adjustments) that it actually collected — a core measure of billing-department effectiveness.

NPI (National Provider Identifier) — A unique 10-digit number assigned to every individual healthcare provider and organization in the U.S. for billing and identification purposes.

O

OIG (Office of Inspector General) — The U.S. Department of Health and Human Services division responsible for investigating fraud, waste, and abuse across federal healthcare programs.

Out-of-Network — A provider without a contract with a given insurance plan; patients typically pay significantly more, or may not be covered at all, for out-of-network care.

Out-of-Pocket Maximum — The most a patient will have to pay for covered services in a plan year, after which the insurance plan covers 100% of allowed costs.

P

Patient Responsibility — The portion of a bill the patient owes after insurance has processed the claim — the sum of any applicable deductible, coinsurance, and copay.

Payer — The organization responsible for paying a claim — an insurance company, Medicare, Medicaid, or, in the case of self-pay, the patient themselves.

Payer Enrollment — The administrative process of registering a provider with a specific payer so that payer will accept and pay claims from them — closely related to, but distinct from, credentialing.

PHI (Protected Health Information) — Any individually identifiable health information that HIPAA requires providers and their vendors to safeguard.

POS (Place of Service) — A two-digit code on a claim identifying where a service was delivered — office, inpatient hospital, telehealth, and so on — which can affect how the service is paid.

PPO (Preferred Provider Organization) — A health plan that contracts with a network of preferred providers at negotiated rates, but still allows patients to see out-of-network providers, usually at higher cost and without needing a referral.

PR (Patient Responsibility) — A claim adjustment group code indicating the amount in question is owed by the patient, not written off by the provider.

Pre-Certification / Preauthorization — Confirmation from a payer, obtained before a service is provided, that the service meets the plan’s coverage criteria.

Premium — The regular (often monthly) payment made to keep an insurance policy active, separate from any costs incurred when care is actually used.

Provider — The general term for anyone delivering healthcare services that can be billed — a physician, therapist, hospital, laboratory, or other licensed entity.

Q

QMB (Qualified Medicare Beneficiary) — A Medicare Savings Program in the U.S. that helps low-income beneficiaries with premiums and cost-sharing.

R

RARC (Remittance Advice Remark Code) — A code that accompanies a CARC on a remittance advice to add further explanation about why a claim line was adjusted.

Referral — A recommendation, usually required by HMO-style plans, sending a patient from their primary care provider to a specialist.

Rejection — A claim returned before it’s ever fully processed, typically because of a technical error (a missing field, an invalid code), as distinct from a denial, which happens after review.

Remittance Advice — The general term for the document (paper or electronic) explaining how a payer processed a claim; see also EOB and ERA.

Revenue Code — A three- or four-digit code used on facility (UB-04) claims to indicate the specific department or type of service a charge is associated with, such as the emergency room or pharmacy.

RVU (Relative Value Unit) — A number reflecting the relative value of a medical service, used by Medicare and other payers to help calculate how much that service is paid.

S

Scrubbing — Reviewing a claim for errors before it’s submitted, to reduce the chance of a rejection or denial.

Self-Pay — A patient paying for care directly, without going through insurance, whether because they’re uninsured or choosing not to use their coverage.

SNF (Skilled Nursing Facility) — A facility providing short-term rehabilitative or nursing-level care, billed under its own set of coding and coverage rules distinct from a hospital or long-term nursing home.

Superbill — An itemized document listing the services, codes, and diagnoses from a patient visit, used as the basis for generating a claim (still common for self-pay patients seeking reimbursement from their own insurer). See our full superbill guide for required fields and common rejection reasons.

T

Taxonomy Code — A 10-character code classifying a provider’s specialty or type of practice, submitted alongside their NPI on claims.

Timely Filing Limit — The deadline by which a claim must be submitted to a payer to be considered for payment at all — commonly a set number of days or months from the date of service, and it varies by payer.

TIN (Tax Identification Number) — The number identifying the billing entity (practice or individual) that will receive payment, used for both billing and tax-reporting purposes.

TRICARE — The U.S. health program covering active-duty military members, veterans, and their families.

U

UB-04 — The standard claim form used by hospitals and other institutional providers — the facility-side counterpart to the CMS-1500.

Upcoding / Downcoding — Upcoding is billing for a more extensive (and more expensive) service than was actually provided; downcoding is a payer reducing a billed code because the documentation doesn’t support the level billed. Both distort payment relative to the actual service delivered, though only one is fraud.

Usual, Customary, and Reasonable (UCR) — A benchmark some payers use to determine a fair allowed amount for a service, based on typical charges for that service in a given geographic area.

Utilization Review — An audit process, sometimes required by a payer, examining whether the services provided to a patient were appropriate and necessary.

V

Value-Based Care — A payment approach that reimburses providers based on patient outcomes and quality of care rather than purely on the volume of services delivered — the opposite end of the spectrum from fee-for-service.

W

Write-Off — An amount permanently removed from a patient account rather than collected, whether because of a contractual adjustment, bad debt, or a charity-care policy.

X

XE, XP, XS, XU Modifiers — A set of four modifiers (XE, XP, XS, XU) that give more specific detail than the older, more general Modifier 59 when reporting distinct procedural services.

Z

Z-Codes — ICD-10-CM codes describing factors that influence health but aren’t themselves an injury or illness — increasingly used to capture social determinants of health, such as housing or food insecurity.