The Collaborative International Dictionary of English v.0.48:

Vanishing \Van"ish*ing\,
   a. & n. from Vanish, v.
   [1913 Webster]

   Vanishing fraction (Math.), a fraction which reduces to the
      form 0/0 for a particular value of the variable which
      enters it, usually in consequence of the existence of a
      common factor in both terms of the fraction, which factor
      becomes 0 for this particular value of the variable.
      --Math. Dict.

   Vanishing line (Persp.), the intersection of the parallel
      of any original plane and the picture; one of the lines
      converging to the vanishing point.

   Vanishing point (Persp.), the point to which all parallel
      lines in the same plane tend in the representation.
      --Gwilt.

   Vanishing stress (Phon.), stress of voice upon the closing
      portion of a syllable. --Rush.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Vanish \Van"ish\ (v[a^]n"[i^]sh), v. i. [imp. & p. p. Vanished
   (v[a^]n"[i^]sht); p. pr. & vb. n. Vanishing.] [OE.
   vanissen, OF. vanir (in comp.): cf. OF. envanir, esvanir,
   esvanu["i]r, F. s'['e]vanouir; fr. L. vanus empty, vain; cf.
   L. vanescere, evanescere, to vanish. See Vain, and cf.
   Evanescent,-ish.]
   [1913 Webster]
   1. To pass from a visible to an invisible state; to go out of
      sight; to disappear; to fade; as, vapor vanishes from the
      sight by being dissipated; a ship vanishes from the sight
      of spectators on land.
      [1913 Webster]

            The horse vanished . . . out of sight. --Chaucer.
      [1913 Webster]

            Go; vanish into air; away!            --Shak.
      [1913 Webster]

            The champions vanished from their posts with the
            speed of lightning.                   --Sir W.
                                                  Scott.
      [1913 Webster]

            Gliding from the twilight past to vanish among
            realities.                            --Hawthorne.
      [1913 Webster]

   2. To be annihilated or lost; to pass away. "All these
      delights will vanish." --Milton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

vanishing
    n 1: a sudden or mysterious disappearance
    2: a sudden disappearance from sight

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "vanishing":
   blackout, blocking, dematerialization, departure, disappearance,
   disappearing, dispersion, dissipation, dissolution, dissolving,
   eclipse, elimination, erasure, evanescence, evanescent,
   evaporating, evaporation, extinction, fadeaway, fadeout, fading,
   fleeting, flying, fugitive, going, melting, occultation, passing,
   transient, vanishing point, wipe