An infix is an affix

Affix

An affix is a morpheme that is attached to a word stem to form a new word. Affixes may be derivational, like English -ness and pre-, or inflectional, like English plural -s and past tense -ed. They are bound morphemes by definition; prefixes and suffixes may be separable affixes...

 inserted inside a word stem

Word stem

In linguistics, a stem is a part of a word. The term is used with slightly different meanings.In one usage, a stem is a form to which affixes can be attached. Thus, in this usage, the English word friendships contains the stem friend, to which the derivational suffix -ship is attached to form a new...

 (an existing word). It contrasts with adfix, a rare term for an affix attached to the end of a stem, such as a prefix

Prefix

A prefix is an affix which is placed before the root of a word. Particularly in the study of languages,a prefix is also called a preformative, because it alters the form of the words to which it is affixed.Examples of prefixes:...

 or suffix

Suffix

In linguistics, a suffix is an affix which is placed after the stem of a word. Common examples are case endings, which indicate the grammatical case of nouns or adjectives, and verb endings, which form the conjugation of verbs...

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In Indonesian (bahasa Indonesia) there are at least 5 kinds of infixes (sisipan). They are -el-, -em-, -er-, -ah-, and -in-.
Examples:

• The word 'gembung' means "bloated", while 'gelembung' means "bubble"'.

• The word 'cerlang' means "luminous", while 'cemerlang' means "brilliant"'.

• The word 'gigi' means "tooth", while 'gerigi' means "serration"'.

• The word 'dulu' means "first" or "advance" or "olden", while 'dahulu' means "formerly"'.

• The word 'kerja' means "work", while 'kinerja' means "performance"'.

English has almost no true infixes (as opposed to tmesis, see below), and those it does have are marginal.