(Indo-European studies) Signifying a nominal stem which alternates between more than one form when declined for grammatical case, particularly in the suffix, most often between *-r- in the strong stem and *-n- in the weak stem(s). Examples of heteroclitic noun stems in Proto-Indo-European include *wod-r/n- (nominoaccusative *wódr̥; genitive *udnés; locative *udén) and *yékw-r/n- (nominoaccusative *yékwr̥, genitive *ikwnés).