(1) The reciprocal action between two entities in a common environment as in chemical interaction, ecologic interaction, and social interaction. (2) The effects when two entities concur that would not be observed with either in isolation. (3) In statistics, pharmacology, and quantitative genetics, the phenomenon that the combined effects of two causes differ from the sum of the effects separately (as in synergism and antagonism). (4) Independent operation of two or more causes to produce or prevent an effect. (5) In statistics, the necessity for a product term in a linear model. (6) The transfer of energy between elementary particles or between fields of energy.