a.

1. 紧要的，关键性的，危急的
   That was a critical time in the nation's history.

   那是涉及国家命运的一个关键时刻。

2. 批评的，批判的，评论性的
   Your critical analysis helped me a great deal.

   你的评论分析对我帮助很大。

3. 吹毛求疵的，爱挑剔的；对……表示不满的[（+of）]
   She is so critical that nobody can get along with her.

   她很爱挑剔，没有人能和她相处得来。

4. 必不可少的；紧缺而必须的
   His attendance is critical to the success of the meeting.

   他的出席对于这次会议的成功是必不可少的。

adj.

1. expressing adverse or disapproving comments or judgements.
2. expressing or involving an analysis of the merits and faults of a work of literature or art.
3. (of a situation or problem) at a point of crisis.
   ▸extremely ill and at risk of death.
   ▸having a decisive importance in the success or failure of something.
4. Mathematics  & Physics  relating to or denoting a point of transition from one state to another.
5. (of a nuclear reactor or fuel) maintaining a self-sustaining chain reaction.