Headshot
From Halopedia, the Halo wiki
A headshot occurs when a bullet or projectile is fired at an enemy's head causing an instant kill. Headshots are most commonly performed with semi-automatic weapons that have VERY HIGH accuracy and high damage levels such as the following:
List of headshot weapons, with headshot power/effectiveness rating against shielded enemys, and flesh (1 to 10, 10 being the strongest)
Sniper Rifle headshot power/effectiveness against shielded enemy: 10 (pure power)
headshot power/effectiveness against flesh: 10 (pure power)
M6D headshot power/effectiveness against shielded enemy: 10 (pure power + fast rate of fire)
headshot power/effectiveness against flesh: 10 (pure power)
M6G headshot power/effeciveness against shielded enemy: 9 (pure power)
headshot power/effectiveness against flesh: 10 (pure power)
M6C Magnum headshot power/effectiveness against shielded enemy: 8 (extrmely fast rate of fire makes up for some of the power)
headshot power/effectiveness against flesh: 10 (pure power)
Beam Rifle headshot power/effectiveness against shielded enemys: 10 (pure power)
headshot power/effectiveness against flesh: 10 (pure power)
BR55HB SR Battle Rifle headshot power/effectiveness against shielded enemys: 8 (power)
headshot power/effectiveness against flesh: 9 (pure power)
Covenant Carbine headshot power/effectiveness against shielded enemys: 8 (fast rate of fire makes up for some of the power)
headshot power/effectiveness against flesh: 10 (pure power)
BR55 Battle Rifle headshot power/effectiveness against shielded enemys: 9 (power)
headshot power/effectiveness against flesh: 10 (pure power)
Headshots require a great amount of accuracy, so using weapons that do not aim well (such as the Assault Rifle). A one-hit-kill can be made with the Shotgun (body or head), although it is difficult and takes practice.
Headshots are most commonly executed by more skilled players than the average rookie, due to the high level of skill involved in shooting someone in the head while running as opposed to a killing shot to the chest such as the Rocket Launcher would provide.
Note that the Assault Rifle and the SMG don't provide any increased damage for headshots, along with many other fully automatic weapons such as the Plasma Rifle.
When the crosshairs of headshot-capable weapons are over an enemy's head, they turn bright red and a small red dot appears in the middle of the crosshairs.
