suppletion

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.