Baron, R.M. & Kenny, D.A. (1986). The Moderator-Mediator variable distinction in Social Psychological research: Conceptual, strategic, and statistical considerations. Journal of Personality and Social Psychology, 51, 1173-1182.
From PsychWiki - A Collaborative Psychology Wiki
Baron & Kenny distinguish between moderator and mediator variables. Prior to the publication of this paper the terms were occassionally used interchangeably within social psychology despite their disinct differences. The authors identify a moderating variable as an interaction variable that affects the strength of the relation between a predictor variable and a dependent variable. A mediating variable directly accounts for a portion of the relationship between a predictor variable and dependent variable.
Baron & Kenny also discuss implications of the distinction between moderator and mediator variables in multiple social psychological contexts and present a series of analytic procedures for effective use of these variables in statistical analysis of behavioral data.