underline

Short definition from Wiktionary

Noun

1. 1
   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.
2. 2
   The character _.
3. 3
   An announcement of a theatrical performance to follow, placed in an advertisement for the current one.
4. 4
   A caption beneath a photograph (which may have more than one line of text).

Verb

1. 1
   To draw a line underneath something, especially to add emphasis.
2. 2
   To emphasise or stress something.
3. 3
   To influence secretly.

Adjective

1. 1
   Passing under a railway line.