Foundations
End of Life
Also: EOL, unsupported software
The point at which maintainers stop releasing fixes, including security fixes, for a version.
In more detail
Flaws found in an end-of-life version are never fixed upstream. A vendor backport can be a short bridge, but the lasting answer is moving to a supported version. Check the end-of-life date before adopting software, and keep an inventory so you know what is about to expire.