1. 叹气;叹息;深呼气
    When you sigh, you let out a deep breath, as a way of expressing feelings such as disappointment, tiredness, or pleasure.

    e.g. Michael sighed wearily...
           迈克尔疲惫地叹了口气。
    e.g. Roberta sighed with relief...
           罗伯塔松了口气。

2. 叹着气说;叹息道
    If you sigh something, you say it with a sigh.

    e.g. 'Oh, sorry. I forgot.' — 'Everyone forgets,' the girl sighed.
           “哦，对不起，我忘了。”——“每个人都有忘事的时候，”那女孩叹着气说道。

3. (风)发出叹息般的声音，悲鸣
    If the wind sighs through a place, it moves through the place with a sound like a sigh.

    e.g. The wind sighed through the valley.
           风在山谷里呼啸而过。

4. 松了口气;如释重负
    If people breathe or heave a sigh of relief, they feel happy that something unpleasant has not happened or is no longer happening.

sigh的解释

    e.g. There was a big sigh of relief once the economic reform plan was agreed...
           经济改革计划一获批准，人们都大大舒了一口气。
    e.g. There was an audible sigh of relief in Washington when the foreign ministers decided to postpone the meeting.
           各国外长决定推迟会议后，华盛顿如释重负。