An abnormality of the base of the skull, which forms the floor of the cranial cavity and separates the brain from other facial structures. The skull base is made up of five bones: the ethmoid, sphenoid, occipital, paired frontal, and paired parietal bones, and is subdivided into 3 regions: the anterior, middle, and posterior cranial fossae. The petro-occipital fissure subdivides the middle cranial fossa into 1 central component and 2 lateral components. [ HPO:curators ]