Melee

From Halopedia, the Halo wiki

Melee: The act of using your weapon as a bludgeon. This can be executed by default by pressing the B button on the Xbox version or the F key on the PC version. Melee attacks are both stealthy and save ammo. One melee attack from behind will kill most enemies, provided you can sneak up behind them. This is called an assassination. If an enemy is hit from the side or front, it may take two or three hits to take them down, depending on which weapon is being used. AI's, such as Elites, Hunters and Combat Forms, will melee if you get too close, but they will almost never attempt a stealth melee. A melee is mostly used when an opponent is too close to effectively shoot, to attempt assassinations, if the person is wielding an Energy Sword, and if the user is low or out of ammunition. Assassinating an opponent gives an Assassin Medal, while simply beating them down wields a Bone Cracker Medal.