Allosaurus (IPA pronunciation: /ˌæləˈsɔɹəs/) is a genus of large (up to 9.7 m long) theropod dinosaurs. The name Allosaurus comes from the Greek allos/αλλος, meaning 'strange' or 'different' and saurus/σαυρος, meaning 'lizard' or 'reptile'. It was named 'different lizard' because its vertebrae were different from those of other dinosaurs known at the time of its discovery. The genus contains three described species, the best known of which is A. fragilis. Several other dinosaurs originally classified as Allosaurus are now in separate genera.
Allosaurus was a bipedal carnivorous dinosaur with a large skull balanced by a long, heavy tail. Relative to the large and powerful hindlimbs, its three fingered forelimbs were small. It was the most common large predator in the Morrison Formation of what is now North America, 155 to 145 million years ago, in the late Jurassic period. It shared the landscape with several genera of giant sauropods such as Apatosaurus, Diplodocus and Camarasaurus as well as other herbivores such as Stegosaurus and Camptosaurus, all of which may have been potential prey.