Found:

Syncretism

MORPHOLOGY: the phenomenon by which a single (inflected) form corresponds to more than one morphosyntactic description. EXAMPLE: in Ancient Greek, the nominative and vocative of the feminine singular/plural case forms are identical (e.g. khóoraa 'a land', khôoraa 'O, land', khôoray 'lands', khôoray 'O, lands'). The same is true for the nominative and accusative of the neuter singular/plural case forms: dôoron 'house-nom./acc.sg.', dôora 'house-nom./acc.pl.'.