下划线在英语中叫作 underscore。它来源于印刷术中的一个符号，就像一条横线。它的用途是用来为一个词或短语做下划线标记，以突出显示这个词或短语。例如：

The word "underscore" is often written with an underscore, like this: underscore.

In many programming languages, the underscore is used as a special character to represent a space in a variable name. For example, the variable name "first_name" might be used to store a person's first name.

The underscore can also be used in URLs to represent a space. For example, the URL "https://en.wikipedia.org/wiki/Underscore" uses an underscore to separate the words "wiki" and "underscore" in the URL.