Critical Hit and Miss
Critical Hit/Critical Miss: On a natural 20 or natural 1 on the d20, whatever was rolling makes either a critical hit or a critical miss. A critical hit automatically succeeds and, in addition to dealing extra damage as per the weapon that the character is using, either gives the character some bonus or gives the character they hit some sort of penalty against them. A critical miss automatically fails and imparts some penalty to the character who did it. Similar concept for other checks.