Short for grenade. Can also be combined with adjectives to describe different types of grenades:
Smoke nade: Grenade that spews smoke when thrown, typically carried by covert ops for the purpose of concealing their movement.
Frag nade: The 'frag' can waither be taken as fragment (aka shrapnel) or frag as in kill. The frag nade is your typical grenade for the purpose of killing.
Flash nade: When thrown, this grenade gives off el muchos lightos, blinding the enemy. Some variations of the flash nade also make a loud bang to temporarily deafen the enemy.
Rifle nade (or riffnad): Grenade attatched to the end of a gun (typically a sniper rifle without a scope) that can be launched a short distance and explodes on, or shortly after, impact. A primitive grenade launcher.