A line placed underneath a piece of text in order to provide emphasis or to indicate that it should be viewed in italics or (in electronic documents) that it acts as a hyperlink.

(uncommon) The character _.

(dated) An announcement of a theatrical performance to follow, placed in an advertisement for the current one.

(dated) A caption beneath a photograph (which may have more than one line of text).

To draw a line underneath something, especially to add emphasis; to underscore

(figuratively) To emphasise or stress something

(figurative, obsolete) To influence secretly.

Passing under a railway line.