Brontosaurus, a term derived from the Greek words, can be translated as "thunder lizard." It represents a herbivorous sauropod dinosaur genus that inhabited what is now the United States during the Late Jurassic period, approximately 156 to 146 million years ago, specifically during the Kimmeridgian and Tithonian ages within the Morrison Formation, which is presently located in Utah and Wyoming.

The anatomy of Brontosaurus is well-documented, with fossils revealing that it was a colossal, long-necked, and quadrupedal creature equipped with an extended tail that culminated in a whip-like structure. Notably, its cervical vertebrae were exceedingly robust and sturdy, setting it apart from its more lightly built relatives, Diplodocus and Barosaurus. Its forelimbs were short and stout, while its hindlimbs were elongated and thick, supported by a robust shoulder girdle and pelvis.

Various estimates have been proposed regarding its size, with the largest species, B. excelsus, reaching lengths of up to 21–22 meters (69–72 feet) from head to tail and weighing in the range of 15–17 metric tons (17–19 short tons). Brontosaurus belongs to the Diplodocidae family, a group of colossal sauropod dinosaurs that includes some of the longest and largest terrestrial creatures in history, such as Diplodocus, Supersaurus, and Barosaurus. Diplodocids first appeared during the Middle Jurassic, initially in what is now Georgia, and later dispersed to North America during the Late Jurassic period.

Brontosaurus falls under the subfamily Apatosaurinae, which also encompasses Apatosaurus and potentially one or more unnamed genera. As a diplodocid sauropod, Brontosaurus was strictly herbivorous and foraged on a diet of ferns, cycadeoids, seed ferns, and horsetails, typically feeding at ground level as a nonselective browser.