Critical Hit
An especially good hit, usually from the best possible roll, that deals extra damage.
A critical hit (or "crit") is a lucky blow. In many games it happens when the twenty-sided die shows 20 and it deals extra damage. The exact rule depends on the ruleset.
In Fantasy RPG
Fantasy RPG: a natural 20 on an attack roll always hits and the damage dice are rolled twice (2d6 + 3 becomes 4d6 + 3). The bonus is not doubled.