Archaism

In language, an archaism (from the Ancient Greek: ἀρχαϊκός, archaïkós, 'old-fashioned, antiquated', ultimately ἀρχαῖος, archaîos, 'from the beginning, ancient') is the use of a form of speech or writing that is no longer current or that is current only within a few special contexts. 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.

A distinction between archaic and obsolete words and word senses is widely used by dictionaries. An archaic word or sense is one that still has some current use but whose use has dwindled to a few specialized contexts, outside which it connotes old-fashioned language. In contrast, an obsolete word or sense is one that is no longer used at all. A reader encounters them when reading texts that are centuries old. For example, the works of Shakespeare are old enough that some obsolete words or senses are encountered therein, for which glosses (annotations) are often provided in the margins.

Archaisms can either be used 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. Some archaisms called fossil words remain in use within certain fixed expressions despite having faded away in all other contexts (for example, is not used in normal English outside the set phrase vim and vigor).

An outdated form of language is called archaic. In contrast, a language or dialect that contains many archaic traits (archaisms) relative to closely related languages or dialects spoken at the same time is called conservative.

...
Wikipedia