In language, an archaism is the deliberate use of an older form that has fallen out of current use.
Archaisms are most frequently encountered in poetry, law, and ritual writing and speech. Their deliberate use can be subdivided into literary archaisms, which seeks to evoke the style of older speech and writing; and lexical archaisms, the use of words no longer in common use.
In English, one sure indicator of a deliberately archaic style is the contemporary use of the second person singular pronoun thou and its related case and verb forms. Ironically, the word thou fell out of English speech because it was thought abruptly colloquial, like French tu; see, T-V distinction. "Thou" is now seen in current English usage only in literature that deliberately seeks to evoke an older style, though there are also some still-read older works that use thou, especially religious texts like the King James Bible.
The compound adverbs and prepositions found in the writing of lawyers (e.g. heretofore, hereunto, thereof) are usually thought of as archaisms.
Archaic syntax is also typically found in these ritual and legal contexts. (e. g. "With this ring I thee wed.") Archaisms are also used in the dialogue of historical novels in order to evoke the flavour of the period. Some may count as inherently funny words and are used for humorous effect.
Archaisms are kept alive by these ritual and literary uses and by the study of older literature. Should they remain recognised, they can be revived, as the word anent was in this past century.