1. an unseen process, event, or system that exists between a stimulus and a response, between the source and destination of a neural impulse, or between the transmitter and receiver of communications.

2. a person—for example, a lawyer or psychologist—who helps contending parties communicate and reach a compromise. See also divorce mediation.

3. in statistical analyses, an intermediary or intervening variable that accounts for an observed relation between two other variables. For example, a researcher may posit a model involving an independent variable of ability, a mediator of self-efficacy, and a dependent variable of achievement. Thus, ability is hypothesized to influence self-efficacy, which in turn is thought to influence achievement.