1640s, "retention of what is old and obsolete," from Modern Latin archaismus, from Greek arkhaismos, from arkhaizein "to copy the ancients" (in language, etc.); see archaic. Meanin [..]

A word, expression, spelling, or phrase that is out of date in the common speech of an era, but still deliberately used by a writer, poet, or playwright for artistic purposes. For instance, two archai [..]

n. Obsolescence.

use of an older or obsolete form. *Pipit sate upright in her chair Some distance from where I was sitting; T. S. Eliot, "A Cooking Egg"

Use of obsolete or old-fashioned language e.g. 'thee', 'thou' or 'beauteous'.  

(n) the use of an archaic expression

usage of an older, often obsolete form of language

  The use in literature of what is archaic, such as words, expressions, spelling, or construction. Thee, quoth, and methinks are examples of archaic words, terms that are no longer in general use.

The adoption or imitation of archaic words or style. An archaic word, style, etc. * L. Douglas *: He had the fastidiousness, the preciosity, the love of archaisms, of your true decadent.