An underline is a horizontal line immediately below a piece of writing.

In handwriting, we traditionally use underlining to indicate emphasis:

underline for emphasis in handwriting

Underline can be a noun and a verb. You can ask someone "to underline" something. ("Please underline all the errors.") In the above example, we can say that butter is "underlined". We can also use the term "underlining" in a more general sense. ("On most web pages, underlining is reserved for links.)

In typewriting, we can use underlining to show emphasis, and also things like titles of books and films, and names of ships.

underline for emphasis and titles in typewriting

However, in print and computer writing, we use underlining much less, partly because we have bold and italics to do a similar job.

bold for emphasis and italic for titles in print

In addition, with the development of the Internet and world wide web, web pages traditionally use underlining to indicate a link. As such, it is not good practice to use underline on web pages.

Underline is also called underscore, especially in American English.