to breathe out slowly and noisily, expressing tiredness, sadness, pleasure, etc.

She sighed deeply and sat down. 她深叹一口气，坐了下来。

[ + speech ] "I wish he was here," she sighed (= she said with a sigh). “我希望他在这里，”她叹息道。

If the wind sighs, it makes a long, soft sound as it moves through trees.

I lay on my back, listening to the sound of the wind sighing in the trees. 我仰面躺着，听着风穿过树林发出的呜咽声。

a slow, noisy breath

He leaned back in his seat with a sigh. 他向后靠在座位上长出了一口气。

"Ah, you're here," she said and heaved/let out/gave a sigh of relief. “啊，你在这儿，”她说道，如释重负地松了口气。