The OED defines LOL as an interjection "used chiefly in electronic communications… to draw attention to a joke or humorous statement, or to express amusement".

《牛津英语词典》将 LOL 定义为一个叹词，“主要用于电子通信……以引起对笑话和幽默语句的注意，或表示可笑”。

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This 'ah' is an interjection used to express surprise.

啊：语气上扬，表示惊讶。

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The second chapter discussed the problem of defining an interjection.

第二章主要讨论有关感叹词的界定问题。

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