Interfix
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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.
Examples
Formation of compound words
In German, the interfix -s- has to be used between certain nouns in compound words, but not all, such as Arbeitszimmer ("workroom") as opposed to Schlafzimmer ("bedroom"). This originates from the masculine and neuter genitive singular suffix -s.
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, compounds of which the first part comes from an Ancient Greek noun whose stem includes o.
In Swedish, compound nouns are written as one word, and interfixes are very common. -s- is frequently used in this way, as in fabriksarbetare, which consists of fabrik (factory) and arbetare (worker). Examples of other interfixes are -e-, as in when familj and far (family and father) become familjefar, and -a-, when viking and by (viking and village) become vikingaby.
Norwegian is closely related to Swedish and has a similar pattern, but uses interfixing somewhat more moderately. Examples: "arbeid" + "rom" = "arbeidsrom" (workroom), but "fabrikk" + "arbeider" = "fabrikkarbeider" and "familie" + "far" = "familiefar".
In Croatian, interfixes -o- and -e- are obligatory when forming a compound. For example, "brod" + "gradilište" = "brodogradilište" (shipyard), but "kuća" + "pazitelj" = "kućepazitelj" (concierge). Unless an interfix is added, the new-formed word is considered to be a word-joining, such as "zimzelen" ("zima" + zelen", evergreen).