In 1842, palaeontologist Richard Owen coined the term dinosaur. The Greek deinos means "terrible," and sauros means "lizard" or "reptile."

Based on the structure of the hipbones, dinosaurs are classified into two orders:

• Saurischians. These "reptile-hipped" dinosaurs have a pelvis that points forward. They are often long-necked, have large and sharp teeth, long second fingers, and a first finger that point away from the rest of the fingers.
• Ornithischians are beaked herbivores and milder.

Well-known dinosaurs like the Tyrannosaurus rex, Deinonychus and Velociraptor fall into this order. Saurischians are divided into two groups.

• Sauropods are four-legged herbivores. They had long heads, long necks and long tails. Leaf-eating Apatosaurus, Brachiosaurus and Diplodocus fall into this family.
• Theropods are two-legged carnivores. There are some of the most fearsome dinosaurs, including Allosaurus and T. rex.

Ornithischians include horned and frilled Triceratops, spiked Stegosaurus and armoured Ankylosaurus.

They often lived in herds and were prey to the larger species of dinosaurs.

In 2017, a study suggested that this hip-oriented classification was incorrect.

Theropods are likely related to the ornithischian dinosaurs. The theropods and Ornithischia form a newly identified group known as Ornithoscelida. If this is correct, it may explain why both theropods and Ornithischia have feathers, while other dinosaurs don't.