APA Dictionary of Psychology

when an analysis of variance or multiple regression analysis has identified an interaction effect among two independent variables, an examination of the effect of one variable at one level of the other variable. For example, if there were two levels of a factor, a₁ and a₂, and two levels of a second factor, b₁ and b₂, the comparison of a₁ versus a₂ at b₁ would represent one simple-effects analysis; another would be a comparison of a₁ versus a₂ at b₂. In this way, a series of simple-effects analyses can be used to break down an interaction into its component parts.