sublineation

from The Century Dictionary.

• noun An underlining; the line so drawn: usually meaning an instruction to the printer to italicize a word so underlined.

from the GNU version of the Collaborative International Dictionary of English.

• noun A mark of a line or lines under a word in a sentence, or under another line; underlining.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A line drawn underneath text; an underline.