There isn't one single perfect Greek word for "infinity" in the way we understand it mathematically. The ancient Greeks didn't conceive of infinity in the same way modern mathematics does. They had concepts related to the limitless or unbounded, but these were philosophical rather than mathematical.
Words like ἄπειρος (apeiros) are often cited. This word means "boundless," "limitless," or "infinite," but it carries a more qualitative sense of endlessness than a precise mathematical definition. It describes something without end, but not necessarily the concept of a completed infinity.