All living languages have elements at every level that are regarded by their users as obsolescent, obsolete, or archaic. (“Old-fashioned,” while descriptive enough, is not, strictly speaking, a linguistic term.) Dictionaries register this fact when they label certain words or meanings archaic, historically older elements perduring alongside normatively contemporary ones. For instance, the first definition of knave in Webster’s Unabridged prefaces the designation “archaic” to three meanings, namely a. ‘a serving boy’, b. ‘a male servant or menial’, c. ‘a man of humble birth or position’ before proceeding to give its modern definition as ‘a tricky deceitful fellow: rogue: rascal: jack’. (Note that for most American card players at least, the last definition is obsolete.)
Even though the designation ‘archaic’ largely affects the lexicon, it may also extend to phonology and morphology. For instance, older speakers may adhere to pronunciations that were dominant when they first learned them as children but have gone out of general use over the speakers’ lifetimes. Thus the increasingly common American English leveling of the paradigm for the word house, which makes the plural into [háusiz] instead of the traditional [háuziz], is an innovation that bids fair to eventually render the latter an archaism. And speakers who follow the norm in forming the plural of wife and knife may still incorporate the newer form for house, where the stem-final /s/ of the singular does not change to /z/. No speaker of English, however, would use an archaism like kine as the plural of cow except for purposes of stylization.
Some archaisms are fossilized in fixed expressions. For instance, in the injunction attributed to Jesus, “Render unto Caesar the things which are Caesar’s, and unto God the things that are God’s” (“Ἀπόδοτε οὖν τὰ Καίσαρος Καίσαρι καὶ τὰ τοῦ Θεοῦ τῷ Θεῷ,” Matthew 22:21), the contextually archaic preposition unto is sustained into modern English from its origin in the King James Bible. Interestingly, the same situation obtains in the Russian equivalent, “Воздай кесарю кесарево, a Богу Богово,” wherein the form of the possessive adjectives for the substantives in question (‘Caesar’s’, resp. ‘God’s’) is archaic.