microcephalyPronunciation: /ˌmʌɪkrəʊˈsɛfəli, -ˈkɛfəli/Definition of microcephaly noun [mass noun] Medicine abnormal smallness of the head, a congenital condition associated with incomplete brain development. Derivatives microcephalicPronunciation: /-sɪˈfalɪk, -kɛˈfalɪk/ adjective & noun microcephalousPronunciation: /-ˈsɛf(ə)ləs, -ˈkɛf-/ adjective microcephaly in other Oxford dictionaries Definition of microcephaly in the US English dictionary