The term 'mental illness' is difficult to define.

“精神病”这个词很难下定义。

《牛津词典》

It is difficult to define what makes him so popular.

很难解释清楚什么原因使他如此走红。

《牛津词典》

It's difficult to define the exact nature of the problem.

很难给这个问题确切定性。

《牛津词典》

Accent is something somewhat difficult to define and categorize.

重音是很难定义和分辨的。

耶鲁公开课 - 现代诗歌课程节选

That's a complicated thing that accountants figure out and accountants find it difficult to define the earnings of a company, but they do something and they come up with a number.

会计们会计算这个复杂的问题,他们还会发现公司收益很难定义,但是他们会想办法,得出一个数字

耶鲁公开课 - 金融市场课程节选

For the man who has built a career on being difficult to define, the defining moment still lies ahead.

CNN: Now, Truly the Supreme Leader

The standards are notoriously difficult to define and seem to shift over time.

FORBES: Magazine Article

Most observers believe that aggression will be left out of the court's jurisdiction because it is so difficult to define legally.

ECONOMIST: The UN and war criminals