A critical hit is where an above normal amount 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.
In most Star Wars shooters, such as the Battlefront series a critical hit is achieved by shooting someone/something in its weak point, which, with humanoids, is usually the head and often results in a one-hit kill.