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"