Evanescent, a.
Etymology: [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.
Shades of meaning
Fernald has no entry of its own for evanescent; he lists it under transient.
Where he writes about it:
That which is fleeting is viewed as in the act of passing swiftly by, and that which is fugitive (L. fugio, flee) as eluding attempts to detain it; that which is evanescent (L. evanesco, from e, out, and vanus, empty, vain) as in the act of vanishing even while we gaze, as the hues of the sunset. — from transient