Learner's definition of SKULL
the structure of bones that form the head and face of a person or animal
see picture at human
He had a fractured skull.
(informal) You need to get it into/through your thick skull [=you need to realize/understand] that you can't just sit there and expect everyone else to do your work for you!
(informal) He can't seem to get it through his skull that I don't like him.