Definitions
1.NounA 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.NounThe character _.
3.NounAn announcement of a theatrical performance to follow, placed in an advertisement for the current one.
4.NounA caption beneath a photograph (which may have more than one line of text).
5.VerbTo draw a line underneath something, especially to add emphasis."underline the important words in the text"
6.VerbTo emphasise or stress something.
7.VerbTo influence secretly.
8.AdjectivePassing under a railway line.