A dinosaur was a prehistoric animal that lived during different periods of time.
The word "dinosaur" can refer to any of those big, usually meat-eating or plant-eating animals that roamed the earth millions of years ago. If someone says they visited a museum and saw a dinosaur fossil, they're talking about an old bone or skeleton of one of these prehistoric creatures.
In scientific usage, any of the animals belonging to the clade Dinosauria, especially those that existed during the Triassic, Jurassic and Cretaceous periods and are now extinct.
