1. A known value or quantity against which an unknown is compared with measured or assessed
2. Information gathered at the beginning of a study from which variations found in the study are measured
3. The initial time point in a clinical trial, just before a participant starts to receive the experimental treatment, which is being tested. Safety and efficacy of a drug are often determined by monitoring changes from the baseline values

See also:
Clinical trial
Placebo
Placebo-controlled studyBias
Blind
Brief Summary
Baseline
Blind (clinical treatment)
Clinical Trial  Enrolement
Clinical
Clinical Trial
Cohort
Control Group
Controlled Trial
Diagnostic Trials
Dichotomous Question
Dose-ranging Trial
Double blind Trial
Drop Out
Drug-drug interaction
Effectiveness
Efficacy
Eligibility Criteria
Endpoint
Experimental Drug
Informed Consent
International Clinical Trial Registry Platform (ICTRP)
Methodology
Open label Trial
Phase I Trials
Phase II Trials
Phase III Trials
Phase IV Trials
Placebo
Placebo Effect
Pre-clinical Trials
Prevention Trials
Quality of Life Trials
Randomisation
Randomised Trial
Risk-benefit analysis
Screening Trials
Single-blind Trial
Trial endpoint
Treatment Trials
Adverse Event
Adverse Reaction