Official Definition
Words or phrases commonly used in an earlier time but rare in present-day usage except to suggest the older time, as in religious rituals or historical novels.
"Archaic." Dictionary.com, www.dictionary.com/browse/archaic. Accessed 6 Sept. 2022.
Informal Definition
Archaic diction is the use of outdated or obsolete language in a piece of literature. This can be a word, phrase, or idea that is not relevant or used in today's society.
"Archaism." Literary Devices, literarydevices.com/archaism/. Accessed 6 Sept. 2022.
Etymology
Archaic comes from the Greek word archaikos which can be literally translated to “from classical Greek culture” or “from the beginning”. Diction is derived from the Latin word dicere which means “to say”.
"Archaic." Wiktionary, en.wiktionary.org/wiki/archaic#:~:text=From%20archaism%20. Accessed 6 Sept. 2022.