- The reciprocal action between two entities in a common environment as in chemical interaction, ecologic interaction, and social interaction.
- The effects when two entities concur that would not be observed with either in isolation.
- 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).
- Independent operation of two or more causes to produce or prevent an effect.
- In statistics, the necessity for a product term in a linear model.
- The transfer of energy between elementary particles or between fields of energy.
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Examples: glitazone, GI cocktail, etc.