Derivation (phonology)

In phonology, derivation refers to either the process or the product of applying a set of phonological rules to an underlying form.

Example
in Dutch we may apply auslautverhaertung, degemination and regressive voicing assimilation in that order to the underlying form [hand][duk], resulting in the form [handuk]. Both the application of these rules and the resulting surface form may be referred to with 'derivation'.

Link
Utrecht Lexicon of Linguistics