An animal or plant becomes extinct when the last of its kind dies. Since the beginning of life on Earth, countless plants, animals, and other organisms have become extinct. In fact, most species that have lived on Earth are now gone. Extinction occurs in the blink of an eye, but factors leading up to it can build up over thousands of years.

Pronunciation: x-TINK-shun
Name Origin: from the Latin extinctus, which means "to extinguish"
Description: Species of plants and animals are evolving every day, and many species are also disappearing, or becoming extinct. Sometimes human activity interferes with and speeds up these processes.
Significance: Extinction is permanent. There's no coming back from being extinct!

Image credits: courtesy of Discovery Channel Online; quote, Mike Novacek courtesy of Discovery Channel Online.