The Collaborative International Dictionary of English v.0.48:

Evanescent \Ev`a*nes"cent\, a. [L. evanescens, -entis, p. pr. of
   evanescere.]
   1. Liable to vanish or pass away like vapor; vanishing;
      fleeting; as, evanescent joys.
      [1913 Webster]

            So evanescent are the fashions of the world in these
            particulars.                          --Hawthorne.
      [1913 Webster]

   2. Vanishing from notice; imperceptible.
      [1913 Webster]

            The difference between right and wrong, in some
            petty cases, is almost evanescent.    --Wollaston.
      [1913 Webster]

WordNet (r) 3.0 (2006):

evanescent
    adj 1: tending to vanish like vapor; "evanescent beauty"

Moby Thesaurus II by Grady Ward, 1.0:

65 Moby Thesaurus words for "evanescent":
   atomic, brittle, capricious, changeable, corpuscular, corruptible,
   deciduous, disappearing, dissolving, dying, embryonic, ephemeral,
   evaporating, fading, fickle, fleeting, flitting, fly-by-night,
   flying, fragile, frail, fugacious, fugitive, germinal, granular,
   impalpable, imperceptible, impermanent, impetuous, imponderable,
   impulsive, inappreciable, inconstant, indiscernible, infinitesimal,
   insubstantial, intangible, invisible, melting, microcosmic,
   microscopic, molecular, momentary, mortal, mutable, nondurable,
   nonpermanent, passing, perishable, short-lived, subatomic,
   temporal, temporary, tenuous, thin, transient, transitive,
   transitory, ultramicroscopic, undurable, unenduring, unseeable,
   unstable, vanishing, volatile