Interfix is a term in linguistics and more specifically, morphology (the study of morphemes, the most basic meaningful entities in word formation). It describes an affix which is placed in between two other morphemes and does not have a semantic meaning. Unlike other affixes, like prefixes (who precede morphemes they are attached to) or suffixes (who are placed after the morpheme they are attached to), an interfix is attached to two different morphemes and thus differs from seemingly similar infix, which is inserted in the middle of one morpheme.

In English, when technical compound words are formed from non-technical roots, an -o- interfix is sometimes used, as o has come to be seen as a connecting vowel: Speed-o-meter, mile-o-meter, by analogy to tacho-meter, odo-meter, and nonsense words such as obscen-o-meter.