A critical hit is where an above normal ammount of damage is inflicted due to hitting a weak point or similar area.
Knights of the Old Republic[]
In the Star Wars: Knights of the Old Republic series, a critical hit is when the random number generator finds the number 20 without adding any bonuses (a 'natural' 20). It uses the d20 system.
Shooters[]
In most Star Wars shooters, such as the Battlefront series a critical hit is achieved by shooting somone/something in its weak point, which, with humanoids, is usually the head and often results in a one-hit kill.
Vehicles[]
Vehicles also have critical hit locations. For the full article see List of Critical Points.