n.[U]

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

   他们为增强体质而做体操。

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

   关于他指挥军队是否适合

   No one questions her fitness for the job.

   没人怀疑她能胜任这项工作。

n.

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