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 with scopes that have a high damage level such as the Sniper Rifle, Particle Beam Rifle, Battle Rifle, Carbine and Pistol.
Headshots require a great ammount of accuracy, so using weapons that do not aim well (such as the Shotgun or Assault Rifle) or weapons that do not have a scope do not regularly result in headshots unless at close range. A one-hit-kill can be made with the Shotgun, although it is extremely difficult and takes a LOT of 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.
Headshots are incredibly easy to do with a Battle Rifle or Carbine. When the crosshairs is over a enemy's head, they turn bright red and a small red dot appears in the middle of the crosshairs.
