An underscore or underline is a line drawn under a segment of text. In proofreading, underscoring is a convention that says "set this text in italic type", traditionally used on manuscript or typescript as an instruction to the printer. Its use to add emphasis in modern documents is a deprecated practice.

The underscore character, _, also called a low line, or low dash, originally appeared on the typewriter so that underscores could be typed. To produce an underscored word, the word was typed, the typewriter carriage was moved back to the beginning of the word, and the word was overtyped with the underscore character.

In modern usage, underscoring is achieved by markup or with the Unicode combining low line. The underscore character is used in strings where spaces are not permitted to create visual spacing, such as in computer filenames, email addresses, and in Internet URLs, for example Mr_John_Smith. It is also used as markup to indicate underscore or italics, for instance _thus_ rendering as thus or thus.