Glossary
Plain-English definitions for the acronyms and technical terms you'll see on hospital pages. Every definition here is drawn from the published CMS and CDC documentation.
- 30-day mortality
- The share of patients who die within 30 days of admission for a specific condition (heart attack, heart failure, pneumonia, COPD, or stroke). Risk-adjusted for patient case-mix.
- 30-day readmission
- When a patient discharged from the hospital is admitted again within 30 days. CMS risk-adjusts readmission rates so hospitals treating sicker patients are compared fairly.
- C. difficile (C. diff)
- Clostridioides difficile - a bacterium that causes serious diarrheal illness, often after antibiotic use in the hospital.
- Case mix
- The mix of patient conditions and severities a hospital treats. Comparing raw scores between hospitals with very different case mixes can be misleading - most CMS outcome measures are risk-adjusted for exactly this reason.
- CAUTI
- Catheter-associated urinary tract infection - a urinary tract infection tied to indwelling urinary catheter use.
- CLABSI
- Central-line-associated bloodstream infection - an infection that develops when a central venous catheter becomes contaminated. One of the CDC's tracked healthcare-associated infections.
- CMS (Centers for Medicare & Medicaid Services)
- The federal agency that runs Medicare, oversees state Medicaid programs, and publishes the hospital quality data used on this site. CMS Provider Data Catalog →
- Denominator / Sample
- The number of eligible cases a measure is based on. Larger samples produce more statistically reliable scores; very small denominators can swing widely release to release.
- ED throughput
- Emergency-department timing measures, including median time from ED arrival to departure for admitted and discharged patients.
- HCAHPS
- Hospital Consumer Assessment of Healthcare Providers and Systems - the standardized patient-experience survey CMS uses. Covers communication with nurses and doctors, responsiveness, cleanliness, pain management, and whether patients would recommend the hospital.
- MRSA
- Methicillin-resistant Staphylococcus aureus - a strain of staph bacteria resistant to several common antibiotics. CMS tracks hospital-onset MRSA bloodstream infections.
- Overall star rating
- A 1–5 summary rating CMS calculates by combining up to 47 quality measures across five categories: mortality, safety, readmission, patient experience, and timely & effective care. Not every hospital receives a star - some report too few measures.
- PSI-90 (Patient Safety Indicator composite)
- AHRQ's composite of serious, potentially preventable in-hospital complications - pressure ulcers, iatrogenic pneumothorax, postoperative sepsis, and others.
- Risk adjustment
- A statistical method that accounts for patient characteristics (age, other diagnoses, severity) so hospitals can be compared on outcomes without penalizing those that treat sicker patients.
- Sepsis bundle (SEP-1)
- The share of severe-sepsis and septic-shock patients who received the recommended bundle of care (labs, antibiotics, fluids) within the target time window.
- SIR (Standardized Infection Ratio)
- The ratio of infections actually observed at a hospital to the number that would be predicted based on a national baseline. An SIR below 1.0 is better than expected; above 1.0 is worse.
- SSI (Surgical Site Infection)
- An infection at or near the surgical incision within 30 days of the procedure (up to 90 days for some implant procedures).
Last reviewed: 2026-07-14.