An interaction effect refers to the role of a variable in an estimated model, and its effect on the dependent variable. A variable that has an interaction effect will have a different effect on the dependent variable, depending on the level of some third variable. For example, increasing doses of a hypertension drug might yield increasing reductions in blood pressure, but the benefit with progressively larger doses might be progressively greater for smokers than for non-smokers.
See also Additive effect.