Abnormal thickness of the skull.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
Examples from the Web for pachycephaly
Pachycephaly (thickness of head) is due to synostosis of the parietal bone with the occipital.Degeneracy|Eugene S. Talbot