Vanishing, or apt to vanish or be dissipated, like vapor; passing away; fleeting: as, the pleasures and joys of life are evanescent.Lessening or lessened beyond the reach of perception; impalpable; imperceptible.In natural history, unstable; unfixed; hence, uncertain; unreliable: applied to characters which are not fixed or uniformly present, and therefore are valueless for scientific classification.In entomology, tending to become obsolete in one part; fading out: as, antennal scrobes evanescent posteriorly.In mathematics, infinitesimal.