Archaic, a.
Etymology: [Gr.
Of or characterized by antiquity or archaism; antiquated; obsolescent.
Shades of meaning
Fernald has no entry of its own for archaic; he lists it under obsolete.
Where he writes about it:
A word is obsolete which has quite gone out of reputable use; a word is archaic which is falling out of reputable use, or, on the other hand, having been obsolete, is taken up tentatively by writers or speakers of influence, so that it may perhaps regain its position as a living word; a word is rare if there are few present instances of its reputable use. — from obsolete