infix

Like prefixes and suffixes, infixes are part of the general class of affixes ("sounds or letters attached to or inserted within a word to produce a derivative word or an inflectional form"). Infixes are relatively rare in English, but you can find them in the plural forms of some words. For example, cupful, spoonful, and passerby can be pluralized as cupsful, spoonsful, and passersby, using "s" as an infix. Another example is the insertion of an (often offensive) intensifier into a word, as in fan-freakin'-tastic. Such whole-word insertions are sometimes called infixes, though this phenomenon is more traditionally known as tmesis.