Infix

An infix, which is rare in English, is a type of affix: that is, a word-fragment which only makes sense when it is added to the middle of another word. (More technically, it is a bound morpheme.) More commonly, in spoken English at any rate, is the phenomenon of tmesis, which is the insertion of a whole word between the beginning and the end of another word or phrase. Do not be tempted to use tmesis in academic writing.