evanescent

Concise Oxford English Dictionary © 2008 Oxford University Press:
evanescent/iːvəˈnesnt/

▶adjective

• 1 chiefly literary quickly fading from sight, memory, or existence.

• 2 Physics denoting a field or wave which extends into a region where it cannot propagate and whose amplitude therefore decreases with distance.

– derivatives
evanesce verb,
evanescence noun,
evanescently adverb.

– origin C18: from L. evanescent-, evanescere ‘disappear’.