Melee

From Halopedia, the Halo wiki

Melee: This is where you use your weapon as a bludgeon and 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 almost any enemies, provided you can sneak up behind them. If an enemy is hit from the side however, it may take two or three hits to take them down. AI's, such as Elites, Hunters and Combat Forms will melee if you get too close. They will almost never stealth melee you, however.