An infix is a morpheme which inserts sounds between the sounds of another morpheme. This is not uncommon in Semitic languages, where roots are three consonants and they are conjugated by changing the vowels and sometimes inserting consonants between them.
English also has infixes, but only in "Massa-fucking-chusetts" and "unbe-fucking-lievable]] and other profanity with the same rhythm. As a rule, these are emphatic markers follow a phonological pattern of syllabic and lexical stress.
In the syntax of notations used in mathematics and computer science, infix is used of operators such as the usual addition sign +, which are taken to bind to the variables immediately preceding and following them. See operator for more on the placement of operators.