pachycephalosaurEnglishOxford New American Dictionarypachycephalosaur /ˌpakiˈsefələsôr, ˌpækɪˈsɛfələsɔr / ▸ noun a bipedal herbivorous dinosaur of the late Cretaceous period, with a thick domed skull. Infraorder Pachycephalosauria, order Ornithischia: several genera, including Pachycephalosaurus. – ORIGIN from Greek pakhus ‘thick’ + kephalē ‘head’ + sauros ‘lizard’.