Guide

Clinical trial glossary

Last reviewed 27 October 2026 · Editorial policy

The words you'll see in study listings and consent forms, explained in plain English.

Term	What it means
Adverse event	Any unwanted health problem that happens during a study, whether or not the study treatment caused it. Study teams must record and report these.
Arm	A group within a study. Each arm gets a different treatment, dose or placebo.
Blinded study	Participants don't know which arm they're in. In a double-blind study, the study team doesn't know either. This keeps results fair.
Clinical trial	A research study that tests a treatment, device or approach in people to see if it is safe and works.
Compensation	Money a study pays for your time and effort. It is separate from reimbursement for travel or parking.
Control group	The participants who get the standard treatment or a placebo, so researchers can compare results.
Eligibility criteria	The rules for who can join, split into inclusion criteria (who can take part) and exclusion criteria (who can't).
Healthy volunteer	Someone without the condition being studied who takes part, often to help test safety or set a baseline.
Informed consent	The process of learning about a study before you agree to join. It continues for as long as you're in the study.
Institutional Review Board (IRB)	An independent committee that reviews and approves a study before it starts, and keeps checking it to protect participants.
Phase	The stage a treatment has reached in testing, from Phase 1 (first in people, mainly safety) to Phase 4 (after approval).
Placebo	A look-alike with no active ingredient, used to compare against the real treatment.
Principal investigator	The doctor or scientist who leads the study at a site.
Protocol	The study's detailed plan: who can join, what happens at each visit, and how results are measured.
Randomized	Participants are assigned to an arm by chance, like a coin toss, rather than choosing.
Recruiting	The study is currently looking for participants.
Screening visit	The first check-up after you consent, to confirm you meet the eligibility criteria.
Study site	The clinic, hospital or research center where study visits happen.