1science, biology one of many types of reptile that lived millions of years ago2something very large and old-fashioned that does not work well or effectively anymore: The Maine dam is a dinosaur which should be removed.3informal an insulting way of describing someone who is old and does not have modern ideas: Some of the dinosaurs of heavy metal music will be on tour this summer. [Origin: 1800–1900 Greek deinos terrible + sauros lizard]