A headshot is a shot received to the head. These shots will always cause 45% more damage and in the case of many sniper rifles, an instant kill. Note that a headshot is different from a critical. Upon scoring a headshot against a target, an orange text saying "Headshot!" will appear, notifying the player or bot who scored a headshot.
Many weapons throughout the series have an increased damage multiplyer upon inflicting a headshot, mostly sniper rifles.