reaction

UK: /riˈækʃ^ən/ US: /riˈækʃən/ ,(rē ak′shən)

WordReference Random House Learner's Dictionary of American English © 2025

re•ac•tion /riˈækʃən/   n. 

1. an action in a reverse direction or manner:
   [countable] For every action there is an equal and opposite reaction.
2. Government [uncountable] a tendency to return to an earlier political system or social order.
3. action in response to some influence, event, etc.:
   [uncountable] the nation's reaction to the president's speech.
   [countable] My reactions are mixed.
4. Medicine [countable]
   • Medicine a response by the body to an action or condition.
   • a change in the body that shows sensitivity to foreign matter:
     an allergic reaction to cat hair.
5. Chemistry [countable] the action of chemical agents upon each other.

WordReference Random House Unabridged Dictionary of American English © 2025

re•ac•tion  (rē ak′shən), n. 

1. a reverse movement or tendency;
   an action in a reverse direction or manner.
2. Government movement in the direction of political conservatism or extreme rightism.
3. action in response to some influence, event, etc.:
   the nation's reaction to the President's speech.
4. Physiology action in response to a stimulus, as of the system or of a nerve, muscle, etc.
5. Medicine
   • Medicine the action caused by the resistance to another action.
   • Medicine a return to the opposite physical condition, as after shock, exhaustion, or chill.
6. Microbiology, Immunology [Bacteriol., Immunol.] the specific cellular response to foreign matter, as in testing for allergies.
7. Chemistry the reciprocal action of chemical agents upon each other;
   chemical change.
8. Physics Also called nuclear reaction. a process in which a nucleus that is bombarded by a photon, particle, or other nucleus, emits a nucleon, alpha particle, or the like, without a significant change in its atomic weight.
9. Mechanics the instantaneous response of a system to an applied force, manifested as the exertion of a force equal in magnitude but opposite in direction to the applied force.
10. Business [Com.] a decline in the market after an advance in prices.

• re- + action, modeled on react 1635–45

re•ac′tion•al, adj. 
re•ac′tion•al•ly, adv. 

Collins Concise English Dictionary © HarperCollins Publishers::

reaction / rɪˈækʃən/ noun

1. a response to some foregoing action or stimulus
2. the reciprocal action of two things acting together
3. opposition to change, esp political change, or a desire to return to a former condition or system
4. a response indicating a person's feelings or emotional attitude
5. any effect produced by the action of a drug, esp an adverse effect
   Compare side effect
6. any effect produced by a substance (allergen) to which a person is allergic
7. the simultaneous equal and opposite force that acts on a body whenever it exerts a force on another body
8. short for chemical reaction, nuclear reaction

reˈactional adjective

USAGE
Reaction is used to refer both to an instant response (her reaction was one of amazement) and to a considered response in the form of a statement (the Minister gave his reaction to the court's decision). Some people think this second use is incorrect