What is "suppletion"

n. 1 The supplying of something lacking. 2 (context grammar English) 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 (context grammar English) 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.

In linguistics and etymology, suppletion is traditionally understood as the use of one word as the inflected form of another word when the two words are not cognate. For those learning a language, suppletive forms will be seen as "irregular" or even "highly irregular". The term "suppletion" implies that a gap in the paradigm was filled by a form "supplied" by a different paradigm. Instances of suppletion are overwhelmingly restricted to the most commonly used lexical items in a language.