An affix is a bit of a word that, incomplete on its own, when put together with another word or bit of word, forms a new word. The plural of affix is affixes, but Nick likes affices, so there. Affixes can go on the front of a word, in the middle of a word, or on the end of a word, just like your mother likes it.
We’re fond of making spurious affices and stems. Among the more useful is the concept of the “quel,” which refers to an iteration of some kind. We have pre-quels, in-quels, and post-quels, also called, archaically, “sequels.”