n.

1. 反應，感應[C][U][（+to）]
   What was his reaction to the plan?

   他對這個計畫有什麼反應？

2. 【化】【物】反應，反作用[C][U]
   Putting an acid and a metal together causes a reaction.

   將酸和金屬放在一起會引起反應。

3. 反動；復古；倒退[U][S][（+from/against）]
   His efforts were defeated by the forces of reaction.

   他的努力被反動勢力挫敗。

4. （興奮過度）疲倦，虛脫，體力不濟[U][S]

n.

1. an instance of reacting to or against something.
   ▸ (reactions) a person's ability to respond physically and mentally to external stimuli.
2. opposition to political or social progress or reform.
3. a process in which substances interact causing chemical or physical change.
4. Physics  a force exerted in opposition to an applied force.