What is a Dinosaur?

A Dinosaur is a creature that has very specific features. It is related to other animals by some of its features but some are unique to them.

A Dinosaur is a Tetrapod - it has a backbone - so do mammals and fishes.

A Dinosaur is a Reptile - Reptiles are cold blooded, lay eggs and breath air (mammals are warm blooded and produce milk to feed their young, fish don't breath air).

A Dinosaur is a Diapsid - A diapsid has two holes on the sides of the skull called temporal fenestra (openings). Turtles and marine reptiles are anapsids with only one opening.

So, a Dinosaur is a Tetrapod, a Reptile and a Diapsid. But Dinosaurs can be divided further into two groups by the shape of their hip bone:

Ornithischian (Bird Hipped)

Saurischian (Lizard Hipped)