In language, an archaism is a word, a sense of a word, or a style of speech or writing that belongs to a historical epoch beyond living memory, but that has survived in a few practical settings or affairs. Lexical archaisms are single archaic words or expressions used regularly in an affair (e.g. religion or law) or freely; literary archaism is the survival of archaic language in a traditional literary text such as a nursery rhyme or the deliberate use of a style characteristic of an earlier age—for example, in his 1960 novel The Sot-Weed Factor, John Barth writes in an 18th-century style. Archaic words or expressions may have distinctive emotional connotations—some can be humorous (forsooth), some highly formal (What say you?), and some solemn (With thee do I plight my troth). The word archaism is from the Ancient Greek: ἀρχαϊκός, archaïkós, 'old-fashioned, antiquated', ultimately ἀρχαῖος, archaîos, 'from the beginning, ancient'.

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, vim 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An outdated form of language is called archaic.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.