It's difficult to define the exact nature of the problem.
很难给这个问题确切定性。
We need to define the task ahead very clearly.
我们需要明确今后的'任务。
The powers of the police must be clearly defined.
警察的职权必须明确界定。
It is difficult to define the border between love and friendship.
爱情和友情之间的界线难以划清。
These categories are not well defined.
这些类别划分得不太明确。