Natural 20

Also known as a "Crit", a "Critical hit", or a "critical success". The best roll that can happen on a 20 sided die: a 20 on the die. This means that, if rolled for an attack, the attack automatically hits as well as does double damage. This has no real special meaning for anything other than an attack, but some games allow this to mean that what ever the character is trying to do automatically succeeds.

This is counter to a Natural 1