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.

The character _.

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

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

“underline the important words in the text”

To emphasise or stress something

To influence secretly.

Passing under a railway line.