From Halopedia, the Halo wiki

Time-to-kill, or TTK for short, is a term in video games that refers to the amount of time it takes for a given weapon to kill an enemy, usually an enemy player in multiplayer. It is calculated assuming that the weapon is fired at its maximum rate of fire, and that every shot hits the target. Projectile travel time is typically ignored, as it varies based on the distance of the enemy. For precision weapons and weapons which deal extra damage on headshots, the time to kill will vary based on whether the player fires at the enemy's head or body.