A liner or lining that goes underneath.

One who underlines.

plural of underliner

A lining on the inside of a garment.

An act or instance of marking text with an underline.

present participle of underline

plural of underline

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 _.

To draw a line underneath something, especially to add emphasis; to underscore / (figuratively) To emphasise or stress something / (figuratively, obsolete) To influence secretly.

Passing under a railway line.