pach·y·ceph·a·lo·saur·i·an 
(păk′ĭ-sĕf′ə-lə-sôrē-ən)
(păk′ĭ-sĕf′ə-lə-sôrē-ən)n.
Any of various herbivorous bipedal ornithischian dinosaurs of the group Pachycephalosauria of the Cretaceous Period, having a very thick skull and a beaked mouth. Also called pachycephalosaur.
[From New Latin Pachycephalosauria, group name, from Pachycephalosaurus, type genus; see PACHYCEPHALOSAURUS.]
pach′y·ceph′a·lo·sauri·an adj.