The Collaborative International Dictionary of English v.0.48:

Twill \Twill\ (tw[i^]l), v. t. [imp. & p. p. Twilled
   (tw[i^]l); p. pr. & vb. n. Twilling.] [Scotch tweel;
   probably from LG. twillen to make double, from twi- two; akin
   to AS. twi-, E. twi- in twilight. See Twice, and cf.
   Tweed, Tweel.]
   To weave, as cloth, so as to produce the appearance of
   diagonal lines or ribs on the surface.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Twill \Twill\, n. [Scotch tweel. See Twill, v. t.]
   1. An appearance of diagonal lines or ribs produced in
      textile fabrics by causing the weft threads to pass over
      one and under two, or over one and under three or more,
      warp threads, instead of over one and under the next in
      regular succession, as in plain weaving.
      [1913 Webster]

   2. A fabric woven with a twill.
      [1913 Webster]

   3. [Perhaps fr. quill.] A quill, or spool, for yarn.
      [1913 Webster]

WordNet (r) 3.0 (2006):

twill
    n 1: a weave used to produce the effect of parallel diagonal
         ribs [syn: twill, twill weave]
    2: a cloth with parallel diagonal lines or ribs
    v 1: weave diagonal lines into (textiles)

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "twill":
   braid, crease, crimp, crisp, dog-ear, double, double over, enfold,
   enlace, entwine, flounce, flute, fold, fold over, frill, gather,
   infold, interfold, interknit, interlace, intertie, intertissue,
   intertwine, intertwist, interweave, intort, knit, lace, lap over,
   loom, loop, mat, net, noose, plait, plat, pleach, pleat, plicate,
   ply, quill, raddle, ruff, ruffle, splice, tissue, tuck, turn over,
   twine, twist, wattle, weave, web, wreathe