suppletion

noun

1. 1. the occurrence of an allomorph of a morpheme which has no phonological similarity to the other allomorphs (Ex.: the -en of oxen, as opposed to a form ending in -s)
   2. the use of an unrelated word (suppletive form) in a paradigm (Ex.: went for the past tense of go)
2. the morphological process by which such replacement occurs

Origin: ME supplecioun < ML suppletio < L suppletus: see suppletory