T. rex (t?? r?ks?)n. A very large carnivorous tyrannosaurid dinosaur (Tyrannosaurus rex) of the Cretaceous Period, having a large head and teeth and short forelimbs with two claws. Also called tyrannosaurus.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.