An "extinct species" is a species of organism that can no longer be found in the wild or in captivity. A species is a classification of organisms which can reproduce successfully with one another.
Once a species has gone extinct, it never reappears on Earth again. A species can go extinct because of several reasons: its habitat has been destroyed, its primary prey has gone extinct, it has been hunted to extinction or some type of catastrophe has devastated its lifestyle in such a short period of time that it is not able to adapt to the changes. Throughout Earth's history, five mass extinction events have occurred, ridding Earth of most of its species during that time period.