noun
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).
verb
To draw a line underneath something, especially to add emphasis.
(figuratively) To emphasise or stress something.
(figurative, obsolete) To influence secretly.
adjective
Passing under a railway line.