Qué es reaction? Qué es reaction? A response to a stimulus.The state resulting from such a response.A reverse or opposing action. A tendency to revert to a former state.Opposition to progress or liberalism; extreme conservatism.Chemistry A change or transformation in which a substance decomposes, combines with other substances, or interchanges constituents with other substances.Physics A nuclear reaction.Physics An equal and opposite force exerted by a body against a force acting upon it.The response of cells or tissues to an antigen, as in a test for immunization.