any of numerous extinct terrestrial reptiles of the Mesozoic era
"The massive fossil found in the excavation was identified as a dinosaur, confirming it belonged to the group of extinct terrestrial reptiles from the Mesozoic era."
In scientific usage, any of the animals belonging to the clade Dinosauria, especially those that existed during the Triassic, Jurassic and Cretaceous periods and are now extinct.
"Paleontologists classify the T-Rex as a theropod dinosaur within the clade Dinosauria because it lived during the Late Cretaceous period before going extinct."
In plain English: A dinosaur is an extinct reptile that lived on Earth long before humans appeared and ranged from tiny creatures to massive giants.
"My little nephew has a stuffed dinosaur in his toy box."
Usage: Use this word to refer exclusively to extinct reptiles from the Mesozoic Era; do not apply it to modern birds despite their evolutionary relationship. Avoid using "dinosaur" as a metaphor for slow or outdated people unless you are intentionally employing irony.
The word dinosaur comes from Ancient Greek words meaning "terrible lizard," which were combined to create the term for these creatures. Paleontologist Richard Owen first coined this name around 1841 or 1842.