Archaism: An archaism is the use of a form of speech or writing that is no longer current. A word or style of expression which has already been outdated. Example of archaism-
Lord, thou hast examined me and knowest me.
Thou knowest all, whether I sit down or rise up:
thou hast discerned my thoughts from afar.
(The Bible, "Psalms"-139)
Here the words "thou" for you, " for know, "hast" for has. A modern writer uses it to add gravity to his meanings.