An ancient, antiquated, or old-fashioned, word, expression, or idiom; a word or form of speech no longer in common use.

Antiquity of style or use; obsoleteness.

(LTC) The use of old or antiquated words in poetry. The device is mainly associated with Spenser's Faerie Queene(1596). Spenser was writing in the late sixteenth century but used and also invented words which struck him as Chaucerian or from the Middle Ages.

n. Obsolescence.

In language, an archaism is the use of a form of speech or writing that is no longer current. This can either be done deliberately (to achieve a specific effect) or as part of a specific jargon (for example in law) or formula (for example in religious contexts). Many nursery rhymes contain archaisms.

the use of an archaic expression