The term 'Critical' is used to refer to either an extreme success 'A Critical Success' or a terrible failure 'A Fumble'. Generally such successes or failures only result from the highest (critical) or lowest (fumble) possible rolls of the dice without modifiers.

In combat a critical usually does extra damage, while a fumble results in injury to the person making the fumble, or the loss of a weapon etc. Similar effects derive from such rolls outside combat.

In addition many systems have specific tables of effects for critical damage in combat. e.g. a critical hit with a an edged weapon might sever a limb.