(used relatively) that; who; which (usually prec. by such or the same).
Ex: ...such a red as makes the eyes ache. <as = pronoun>
If as were a conjunction, and the subject covert (not seen or heard), then the verb would lack tense like this:
Ex: He was so foolish as to lie. <as = conjunction>Note that, when as functions as a conjuction it means "with the result", which our example sentence such a red as makes the eyes ache appears to express; however, if we take a look at the syntax, the verb makes carries tense, which means it has an overt subject--that subject being the pronoun as.