A massive skeleton was found in 1879 by Othniel Marsh, a Professor of Paleontology at Yale University. Marsh incorrectly identified the find as a new species, and he called it Brontosaurus excelsus. What was actually discovered was an adult-sized Apatosaurus, a species that was already known at the time.
A display of the gigantic mounted skeleton (with a Camarasaurus head) cemented the name Brontosaurus (thunder lizard) into the public consciousness.
It wasn’t long before the find was identified correctly and the new name was dropped in the scientific community. However, the public had already latched on to the name Brontosaurus for the famously large creature and the term was used incorrectly for almost a century. In fact, the designation persisted as an official term in literature until at least 1974. Of course, the Flintstones didn’t help much!
Because of the wide use of the term, the words brontosaurus, brontosaurs, and brontosaurians (no capital 'B'; no italics) are often used to refer generically to any of the sauropod dinosaurs. But there is no such dinosaur as Brontosaurus.