1751, "process of gradually vanishing;" see evanescent + -ence. Meaning "quality of being evanescent" is from 1830. Evanescency is attested from 1660s.
Entries linking to evanescence
evanescent adj.
1717, "on the point of becoming imperceptible," from French évanescent, from Latin evanescentem (nominative evanescens), present participle of evanescere "disappear, vanish, pass away," figuratively "be forgotten, be wasted," from assimilated form of ex "out" (see ex-) + vanescere "vanish," inchoative verb from vanus "empty, void" (from PIE *wano-, suffixed form of root *eue- "to leave, abandon, give out"). Sense of "quickly vanishing, having no permanence" is by 1738.
-ence
see -ance.
evanesce v.
"vanish by degrees, melt into thin air," 1817, a back-formation from evanescence, or else from Latin evanescere "disappear, vanish, pass away," figuratively "be forgotten, be wasted," from assimilated form of ex "out" (see ex-) + vanescere "vanish," inchoative verb from vanus "empty, void" (from PIE *wano-, suffixed form of root *eue- "to leave, abandon, give out").