suppletion (n.)

A term used in MORPHOLOGY to refer to cases where it is not possible to show a relationship between MORPHEMES through a general RULE, because the forms involved have different ROOTS. A suppletive is the grammar’s use of an unrelated FORM (i.e. with a different root) to complete a PARADIGM, as in the present–past-TENSE relationship of go ~ went, or the COMPARATIVE form better in relation to good.