“Archaism (noun): Antiquated expression, diction, or style; out-of-date or old-fashioned word or phrase, often having some currency or literary usefulness, e.g., “perchance,” “forsooth,” “betwixt.” Adj. archaism; v. archaize.
‘A part of our reality is the unreality of archaic language about sex.’ Dwight Bolinger, Language—The Loaded Weapon”
Excerpted from: Grambs, David. The Random House Dictionary for Writers and Readers. New York: Random House, 1990.