Definition of: dinosaur

(dī′nə·sôr) noun Paleontol.
One of a group of reptiles (orders Saurischia and Ornithischia) existing on all the continents, but extinct by the end of the Mesozoic period. They varied in size from small, two-footed, pigeonlike carnivores to gigantic, four-footed, aquatic and terrestrial forms, many of them heavily armored, of both herbivorous and carnivorous habits. [<DINO- + Gk. sauros lizard]
—di′no·sau′ri·an adjective & noun