Something that no longer exists and has no living representative is extinct. Dinosaurs are extinct. Polar bears may become extinct. Sadly, history teachers who force their students to memorize long lists of dates are in no danger of becoming extinct.
You often hear of a species being extinct: there are no living representatives of the species left. But extinct also describes something that has gone out or grown cold, as in a dormant volcano "extinct volcano" or a tobacco pipe whose flame has died out "extinct pipe." That’s because extinct comes from the Latin extinguere/exstinguere for "extinguish." Originally extinct referred to fires that died out (were extinguished). Its meaning grew to encompass a family line and then a species.