1. 反应；回应；反响
    Your reaction to something that has happened or something that you have experienced is what you feel, say, or do because of it.

    e.g. Reaction to the visit is mixed...
           对这次访问的反应不一。
    e.g. The initial reaction of most participants is fear...
           多数参与者的最初反应是恐惧。

2. 反对;反抗;抵制
    A reaction against something is a way of behaving or doing something that is deliberately different from what has been done before.

    e.g. All new fashion starts out as a reaction against existing convention...
           所有新时尚都是从打破现有常规开始的。
    e.g. Her three years of freedom seemed like a reaction against that childhood.
           她那3年的自由仿佛是对孩童时代的反叛。

3. 抵制；抵触
    If there is a reaction against something, it becomes unpopular.

    e.g. Premature moves in this respect might well provoke a reaction against the reform...
           在这方面不成熟的举动很可能引发对改革的抵触情绪。
    e.g. The government response seems to be the result of strong public reaction against the police apathy.
           政府的回应似乎是公众强烈抵制警察冷漠执法的结果。

4. 反应力;应变能力
    Your reactions are your ability to move quickly in response to something, for example when you are in danger.

    e.g. If his reactions are so slow, he shouldn't be in charge of a bus...
           如果他的反应太慢，就不应该开公交汽车。
    e.g. The sport requires very fast reactions.
           这项运动要求极快的反应力。

5. 保守思想;反动思想
    Reaction is the belief that the political or social system of your country should not change.

    e.g. Thus, he aided reaction and thwarted progress.
           因此，他帮助反动派阻挠进步。
    e.g. ...their victory against the forces of reaction and censorship.
           他们取得的对保守势力和审查制度的胜利

6. （化学）反应，变化
    A chemical reaction is a process in which two substances combine together chemically to form another substance.

    e.g. Ozone is produced by the reaction between oxygen and ultraviolet light...
           臭氧由氧气和紫外线发生反应而产生。
    e.g. Catalysts are materials which greatly speed up chemical reactions.
           催化剂是可以大大加速化学反应的物质。

7. 过敏;不良反应
    If you have a reaction to a substance such as a drug, or to something you have touched, you are affected unpleasantly or made ill by it.

reaction的近义词

    e.g. Every year, 5000 people have life-threatening reactions to anaesthetics...
           每年有5,000人对麻醉剂产生致命反应。
    e.g. Common foods which cause this kind of reaction are fish, eggs, and shellfish.
           引发这种不良反应的常见食物包括鱼、蛋和贝类。