In
linguistics, archaic generally refers to a
word,
expression, or
construction that is no longer part of the
vernacular, but which is still recognized by speakers of the language as being an
antiquated, older form the modern vernacular.
For example, the word "thou" in English. While it is never used in normal conversation today, almost every English speaker recognizes "thou" as being an archaic form of "you", found in old literature, such as Shakespeare.