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.

A change or transformation in which a substance decomposes, combines with other substances, or interchanges constituents with other substances.

A nuclear reaction.

An equal and opposite force exerted by a body against a force acting upon it.