suppletion

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Linguistics The use of an unrelated form to complete a paradigm, as the past tense went of the verb go, goes, going, gone.

Wiktionary

1. n. The supplying of something lacking.
2. n. The use of an unrelated word or phrase to supply inflected forms otherwise lacking, e.g. using “to be able” as the infinitive of “can”, or “better” as the comparative of “good”.
3. n. More loosely, use of unrelated (or distantly related) words for semantically related words which may not share the same lexical category, such as father/paternal or cow/bovine.