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)
The term infix is first attested in the last quarter of the 19th century.
Plag, Ingo. 2003. English word-formation. Cambridge:Cambridge University Press.
German Infix (de)