The inflection of a word is suppletive iff different inflectional categories trigger an allomorphic variaton of the stem such that the allomorphs are neither phonologically nor etymologically related. This kind of allomorphy is called suppletion. The obtaining of suppletion in a system is called suppletivism.

There are two principal kinds of suppletion:

1. Suppletion as a morphological process: significative suppletion: If a root is put into an inflectional category, the root and the category merge into one morpheme.
2. Suppletion as a kind of allomorphy: allomorphic suppletion: If a root is marked for an inflectional category, it assumes an unrelated form.

Examples:

1. English am and was.
2. English go and wen- (in went) (Matthews 1991:140); Engl. good vs. bett- (in better).

Suppletion of verb roots conditioned by number is found in Khoisan languages.

Concept and term were introduced in Osthoff 1899.
The term 'suppletion' is generally restricted to stem/root allomorphy, although inflectional and derivational morphemes may likewise exhibit allomorphy between unrelated allomorphs.