Archaic means "old fashioned" or "no longer in use." When applied to language, it refers to a word, phrase, or usage that is either no longer used in the language or whose meaning has significantly changed.
Archaism is the noun form, referring to an old-fashioned or obsolete use of a word, phrase, or usage.
Example: The word thou, the singular form of you, is an archaism.
Example: The word prevent has an archaic meaning of "precede;" today it means "hinder."
Some dictionaries use the word obsolete when referring to archaisms.