An underline is a horizontal line immediately below a piece of writing.
In handwriting, we traditionally use underlining to indicate emphasis:
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.
However, in print and computer writing, we use underlining much less, partly because we have bold and italics to do a similar job.
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.