Source : Webster's Revised Unabridged Dictionary (1913)

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.

            So evanescent are the fashions of the world in these
            particulars.                          --Hawthorne.

   2. Vanishing from notice; imperceptible.

            The difference between right and wrong, is some
            petty cases, is almost evanescent.    --Wollaston.

Source : WordNet®

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