Alternatively referred to as a low line, low dash, and understrike, the underscore ( _ ) is a symbol found on the same keyboard key as the hyphen. The underscore is commonly used as an alternative to the space key when the space is not allowed.
For example, in computer programming, a variable may be named $my_variable, using an underscore instead of a space because spaces are not allowed in a variable name.
How to create an underscore
Creating the _ symbol on a U.S. keyboard
Creating the _ symbol on a smartphone or tablet
Underscore and underline
An underscore is different than an underline. If you are trying to underline any letter, number, or other text, the underline feature should be used. The underscore, by contrast, takes the place of another character, like a number, letter, or symbol.