n.

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

   他对这个计划有什么反应？

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

   将酸和金属放在一起会引起反应。

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

   他的努力被反动势力挫败。

4. （兴奋过度）疲倦，虚脱，体力不济[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.