A formative is a exponent of a morpheme. Formatives comprise not anly affixes, phrasal affixes and particles but also non-concatenative changes like umlaut, tone or stress change, deletion of one or more segments, consonant mutation and the like.
The term became well-known in the 1960s through its use in Chomsky & Halle (1968).
English plural morpheme is the formative -s like in cat-s (cat+PL).
The German clause negation particle nicht is a formative. Despite its syntactical separation from V0 grammatically (morphologically) it is a part of the verb inflection.
The term formative was introduced by Karl Brugmann (see Brugmann 1908), as a replacement for suffix, which he thought had unwanted diachronic connotations (Latin suffixum originally means 'attached element' and might suggest that such an element arose from a previously free word by diachronic aglutination).
- Brugmann, Karl. 1908. Formans oder Formativum? Indogermanische Forschungen 22:69-72.
- Chomsky, Noam A. & Halle, Morris. 1968. The sound pattern of English. New York: Harper & Row.