underliner

verb (used with object), un·der·lined, un·der·lin·ing.

1. to mark with a line or lines underneath; underscore.
2. to indicate the importance of; emphasize, as by stressing or italicizing.

noun

3. Printing. a caption under an illustration.
4. a line drawn under something; an underscore.

verb (ˌʌndəˈlaɪn) (tr)

1. to put a line under
2. to state forcibly; emphasize or reinforce

noun (ˈʌndəˌlaɪn)

3. a line underneath, esp under written matter

v.1721, from under + line (v.). Cf. Dutch onderlijnen. Related: Underlined; underlining. The noun is attested from 1888.