noun

1. a return or opposing action, force, influence, etc.
2. a response, as to a stimulus or influence
3. a movement back to a former or less advanced condition, stage, etc.; esp., such a movement or tendency in economics or politics; extreme conservatism
4. Chem.
   1. the mutual action of substances undergoing chemical change
   2. a process that involves changes within the nucleus of an atom
   3. the state resulting from such changes
5. Med.
   1. an action induced by resistance to another action
   2. the effect produced by an allergen
   3. a depression or exhaustion of energy following nervous tension, overstimulation, etc.
   4. an increased activity following depression
6. Physiol., Psychol. an organic response to a stimulus