The Collaborative International Dictionary of English v.0.48:

brontosaur \bron`to*saur"\, n. [NL., fr. Gr. bronth` thunder +
   say^ros lizard.] (Paleon.)
   a dinosaur of the genus Brontosaurus; an individual may
   also be called a brontosaurus or an apatosaurus.
   [PJC]

WordNet (r) 3.0 (2006):

brontosaur
    n 1: huge quadrupedal herbivorous dinosaur common in North
         America in the late Jurassic [syn: apatosaur,
         apatosaurus, brontosaur, brontosaurus, thunder
         lizard, Apatosaurus excelsus]