1. 恶梦;噩梦;梦魇
    A nightmare is a very frightening dream.

    e.g. All the victims still suffered nightmares...
           所有受害者仍然被恶梦困扰。
    e.g. Jane did not eat cheese because it gives her nightmares.
           简没吃奶酪，因为那会让她做恶梦。

2. 令人恐怖的处境;可怕的情景
    If you refer to a situation as a nightmare, you mean that it is very frightening and unpleasant.

    e.g. The years in prison were a nightmare.
           监狱里的那些年是一场噩梦。

3. 恼人的情景;惹人烦的局面
    If you refer to a situation as a nightmare, you are saying in a very emphatic way that it is irritating because it causes you a lot of trouble.

    e.g. Taking my son Peter to a restaurant was a nightmare...
           带我儿子彼得外出吃饭简直糟糕透了。
    e.g. In practice a graduate tax is an administrative nightmare.
           毕业税具体操作起来不啻一场行政噩梦。