If a word is described as archaic, it suggests its use is now old-fashioned. Many words in poems are still used that seem archaic, and many formal words may seem to be so, especially in a religious or legal register. Such words may not be really archaic - it may simply be that you are unaware of these particular registers. Take great care when writing about language in A2 change not to label a word archaic simply because you haven't heard of it - better to say 'formal'.
In literature, an archaism is a deliberate and intentional use of older forms of language. For instance, nineteenth Century poetry retains the use of "thee" and "thou" as second person pronouns, even though they had passed out of use in conventionsl Standard English. If we are looking at an old text which contains words which were current at the time, but have since passed out of use, we can talk about obsolete language.