The Collaborative International Dictionary of English v.0.48:

Underscore \Un`der*score"\, v. t.
   To draw a mark or line under; to underline. --J. Tucker.
   [1913 Webster]

WordNet (r) 3.0 (2006):

underscore
    n 1: a line drawn underneath (especially under written matter)
         [syn: underscore, underline]
    v 1: give extra weight to (a communication); "Her gesture
         emphasized her words" [syn: underscore, underline,
         emphasize, emphasise]
    2: draw a line or lines underneath to call attention to [syn:
       underline, underscore]

Moby Thesaurus II by Grady Ward, 1.0:

103 Moby Thesaurus words for "underscore":
   accent, accentuate, band, bar, belabor, blaze, blaze a trail,
   blemish, blotch, brand, chalk, chalk up, check, check off,
   cicatrize, cross-hatching, dapple, dash, define, delimit,
   delineation, demarcate, diagonal, discolor, dot, dotted line,
   dwell on, emphasize, engrave, feature, fleck, freckle, gash,
   give emphasis to, hachure, hairline, harp on, hatch, hatching,
   highlight, impress, imprint, italicize, line, lineation,
   make a mark, mark, mark off, mark out, mottle, nick, notch,
   overaccentuate, overemphasize, overstress, pencil, pepper,
   place emphasis on, play up, point, point up, prick, print, punch,
   punctuate, puncture, riddle, rub in, scar, scarify, score, scotch,
   scratch, seal, seam, slash, speck, speckle, splotch, spot,
   spotlight, stain, stamp, star, stigmatize, streak, streaking,
   stress, striate, striation, strip, stripe, striping, stroke,
   sublineation, tattoo, tick, tick off, trace, underline,
   underlining, underscoring, virgule



The Free On-line Dictionary of Computing (30 December 2018):

underscore

    _, ASCII 95.

   Common names: ITU-T: underline; underscore; underbar; under.
   Rare: score; backarrow; skid; INTERCAL: flatworm.

   See also left arrow.

   (1995-03-06)