main effect

Definition
Main effect: An effect that occurs in factorial designs when the averages between categories of one factor are different overall (e.g., females averaged over treatment and control groups score differently from males averaged over treatment and control groups). Each factor in a factorial design permits an evaluation of its main effect. The key word here is overall.

A main effect in statistics refers to the impact of an independent variable on a dependent variable, averaged across the levels of other independent variables. It helps to identify how changes in one factor influence the outcome, without considering interactions with other factors. (Wikipedia)
Notes

Main effects compare the overall influences of each factor ( e.g., the average of all males and females for the treatment condition versus the average of all males and females for the control condition ). (Suter 2012, pg. 16)