Critical hit

From TF2 Wiki

A critical hit or crit is a shot that does extra damage, glows your respective team color, and has distinctive sounds when fired and received. When you hit a player with a critical, the words "critical hit!!!" appear over the target's head in green letters (see image).

Sniper headshots are critical hits. The spy's butterfly knife does not have normal critical hits, but instead has backstab (which might be considered a critical hit). A shot is critical when it was fired, so it is possible to miss. Automatic weapons such as the minigun and the flamethrower have short periods where every shot is critical. In general for all non-melee non-automatic weapons (with the exceptions of the sniper's rifle) the base critical chance is 5%. For all melee weapons with the exception of the butterfly knife the base chance of a critical hit is 10%.

Critical hits are awarded based on random chance, starting at 5% for most weapons (except headshots and backstabs.) Damage dealt by a player over the last 20 seconds and increases the crit chance, up to a maximum of 20% [1]. This "super-crit" period can be extended by continued killing. Additional stats on crits have been gathered by testing done by Redcomet and posted on the Steam Users Forums.

Critical hits, including headshots, deal three times as much damage as regular hits. [2] Critical hits do only normal damage to buildings.

During humiliation every attack made by the winning side is a critical hit, including swipes with the butterfly knife.

Any shots that glow your team's colors after being fired (sticky bombs, rockets, shotgun shells, minigun rounds) and have an electric noise to the firing are critical hit shots.

All of your shots are critical while ubercharged with the Kritzkrieg. This affects the 'Blutsauger' gun, which normally doesn't crit. The Kritzkrieg doesn't affect sniper body shots or spy knife stabs.