Part of a word you insert between existing parts of an existing word. In the english language, these are rare, with only two consistantly-usable examples, those being the nefarious duo of "-fuck-" and "-funk-", as illustrated in "fan-FUCKing-tastic!" and "interFUNKonnectedness".

NatchLucid would like me to make some space here for the lizittlest infix of them all, the sizyllable "-iz-." You got a problem wit dat, bizitch? Actually, I might. While expletive in-funkin'-fixation denotes an increase in enthusiasm or intensity, "iz" doesn't seem to modify the meaning of the word at all, like a prefix or suffix might be reasonably expected to. (be reason expect to?) Unfortunately, I am so far removed from my linguistics class as to not recall whether or not infixation necessary and/or exclusively works on the phoneme or morpheme level - or, as in my above "definition", only occurs with chunks of actual words... not just weird and disonnant syllables. So for the time being, I'm going to have to say that thizis not (ha ha) infixation at work until someone can learn me (or conclusively assert below) otherwise. Cheers!