lissencephaly
n. severe malformation of the convolutions (gyri) in the cerebral cortex due to abnormal neuronal migration during development. It is an umbrella term encompassing several conditions: agyria, the complete absence of convolutions; pachygyria (or macrogyria), unusually thick convolutions; and polymicrogyria (or micropolygyria), unusually small but numerous convolutions. Lissencephaly results in severe developmental delay, severe to profound intellectual disability, motor impairment, and epilepsy.