Developmental failure of the cranial sutures to close completely, especially at the occiput, usually leading to grossly defective development of the brain.
Source:
PAC, 2016, 88, 713. 'Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)' on page 740 (https://doi.org/10.1515/pac-2015-1202)