"下划线" in Engpsh is called "underscore".
It comes from the use of the character "_" to indicate a pne under a word, used to indicate emphasis, pnking, or as a visual separator.
Example in Engpsh:
The username must contain an underscore.
The filename "readme_2022.txt" is easier to read with an underscore.
中文翻译：
用户名必须包含下划线。
带有下划线的文件名"readme_2022.txt"更容易阅读。