Evidence-appraisal glossary
Mediator
A mediator is a variable that lies on the causal pathway between an exposure and an outcome, carrying part or all of the exposure's effect. Adjusting for a mediator removes the portion of the effect that travels through it.
Also called: intermediate variable, mediating variable, mediation.
Mediation analysis splits a total effect into a direct part and an indirect part that runs through the mediator, which helps explain how an exposure produces an outcome. A common error is treating a mediator as a confounder and adjusting it away, which hides the very effect you are trying to measure. Whether a variable is a mediator or a confounder depends on the assumed causal structure and the question being asked, not on the data alone.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.