Critical Hits are high damage dealing shots that you can get randomly
. When you have a critical hit, small electric bolts will be visible near you melee weapon or a bullet. When you hit or get hit by someones Critical hit, then a sound will play.

Critical hits are random and every weapon can get random criticals except the Spy's knife which only gets criticals when a backstab is made.

The normal rate of getting critical hits is 5% by default and your chance of getting more critical hits gets higher based on the amount of damage or how good you are doing.

On the Xbox 360 version critical hits can be turned off by the servers host by changing the settings in the options.