Infix

An infix is an affix which occurs inside its base.

Examples
Tagalog, a language spoken at the Philippines, has a number of infixes. From the monomorphemic root sulat 'writing' the derived verb sumulat 'to write' is formed by infixing -um- after the initial consonant. The existence of infixes is not uncontroversial. Broselow &amp; McCarthy (1983) and McCarthy (1986) argue that infixation is just a special kind of prefixation or suffixation.

Comment
Sometimes the term infix is also used for adfixes that occur nonperipherally in a word, but not inside another morpheme. However, this usage of infix is usually regarded as erroneous.
 * "Now, shouldn’t we analyze -al in decolonialization also as an infix (after all, it occurs inside a word)? The answer is ‘no.’ True, -al occurs inside a complex word, but crucially it does not occur inside another morpheme." (Plag 2003:11)

Arabic infixes, sometimes also called transfixes are vocalic patterns within so-called discontinuous morphemes, traditionally called roots. For example, the triconsonantal root {k..t..b} is the discontinuous morpheme, which carries the meaning of 'writing', into which a vocalic pattern such as {..a..a} can be infixed to give you /katab/ (a pausal form), meaning 'wrote'. In fact, the morphemic analysis of past verb forms in Arabic is more complex than it might overtly seem were we to add gender as yet a third morpheme.

Origin
The term infix is first attested in the last quarter of the 19th century.

Link
Utrecht Lexicon of Linguistics

Other languages
German Infix (de)