A Critical Hit is an attack that does more than the usual amount of damage to a monster.
When an attack scores a critical hit, a different animation is used instead of the normal hit animation.
The higher an attacker's DEX, the greater their Critical Hit Rate against an opponent is. The higher a defender's AGI, the less likely they are to have critical hits scored against them.
The minimum critical hit rate from DEX vs. AGI is 5%, the maximum is 20%. Equipment and merits that increase critical hit rate are added on top of this.
It is currently believed that critical rate caps out when DEX is 50 points higher than AGI, at least at level 75.
We can suppose it works like this : Critical Hit rate = (dDEX-25)% with 5%min and 20%max
Items which increase the damage from a critical hit include Brave Grip, Cruadin, and X's Knife.