Real Dictionary

dinosaur
countable noun: Dinosaurs were large reptiles which lived in prehistoric times.
countable noun: If you refer to an organization as a dinosaur, you mean that it is large, inefficient, and out of date.

dinosaur in American English
any of various extinct, mostly land-dwelling, four-limbed saurischian or ornithischian reptiles of the Mesozoic Era, including some c. 30 m (98.4 ft) long: the flesh-eaters usually walked on their hind limbs, the plant-eaters on all fours
noun: any chiefly terrestrial, herbivorous or carnivorous reptile of the extinct orders Saurischia and Ornithischia, from the Mesozoic Era, certain species of which are the largest known land animals

dinosaur in British English
noun: any extinct terrestrial reptile of the orders Saurischia and Ornithischia, many of which were of gigantic size and abundant in the Mesozoic era