n.[U]

1. 健康
   They're doing exercises to improve their fitness.

   他們為增強體質而做體操。

2. 適當，適合[（+for）][+to-v]
   about his fitness to command the army

   關於他指揮軍隊是否適合

   No one questions her fitness for the job.

   沒人懷疑她能勝任這項工作。

n.

1. the state of being fit.
2. Biology  an organism's ability to survive and reproduce in a particular environment.