suppletion

The supplying of something lacking.

(linguistics, grammar) 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”, or “went” as the simple past of “go”.

(grammar) More loosely, the 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.