Nonconcatenative morphology, also called discontinuous morphology and introflection, is a form of word formation and inflection in which the root is modified and which does not involve stringing morphemes together sequentially.

In English, for example, while plurals are usually formed by adding the suffix -s, certain words use nonconcatenative processes for their plural forms:

Many irregular verbs form their past tenses, past participles, or both in this manner:

This specific form of nonconcatenative morphology is known as base modification or ablaut, a form in which part of the root undergoes a phonological change without necessarily adding new phonological material. In traditional Indo-Europeanist usage, these changes are termed ablaut only when they result from vowel gradations in Proto-Indo-European. An example is the English stem s⌂ng, resulting in the four distinct words: sing-sang-song-sung. An example from German is the stem spr⌂ch "speak", which results in various distinct forms such as spricht-sprechen-sprach-gesprochen-Spruch.

Changes such as foot/feet, on the other hand, which are due to the influence of a since-lost front vowel, are called umlaut or more specifically I-mutation.

Other forms of base modification include lengthening of a vowel, as in Hindi:

or change in tone or stress:

Consonantal apophony, such as the initial-consonant mutations in Celtic languages, also exists.