What does archaism mean?
The definition of archaism
Ar”cha*ism, n.
Etym: [Gr. archaïsme. See Arch, a.]

1 An ancient, antiquated, or old-fashioned, word, expression, or idiom; a word or form of speech no longer in common use.
2 Antiquity of style or use; obsoleteness. A select vocabulary corresponding (in point of archaism and remoteness from ordinary use) to our Scriptural vocabulary. De Quincey.