A troop is a military unit, which can have different meanings depending on the country in which it is used.

British Army and Royal Marines

In the British Army the definition of a troop varies by corps.

• Household Cavalry and Royal Armoured Corps: Three or four armoured fighting vehicles commanded by a subaltern, or the equivalent of a platoon in headquarters elements. The troop commander commands one vehicle himself, the troop sergeant a second, and the others are commanded by corporals.
• Special Air Service: Sixteen men, organised into four four-man patrols, and commanded by a captain (who also commands one of the patrols himself, the troop sergeant commanding another).
• Royal Engineers, Royal Corps of Signals and Royal Logistic Corps: A unit equivalent in size to a platoon in other corps, divided into sections.

Other corps do not use the term, although in the Royal Horse Artillery a troop used to be the equivalent to a battery in other artillery units.

In the Royal Marines, a troop is the equivalent to an army platoon.

General military usage

The mass nouns troop or troops can also be used more generally:

• To refer to all active duty military (e.g. Support our troops, Bring the troops home).
• To refer to ground forces generally (e.g. troop leaders, combat troops).

When referring to a specific service different words are used (singular or plural):

• Royal Navy/United States Navy - Sailor
• Royal Air Force/United States Air Force - Airman
• British Army/United States Army - Soldier
• Royal Marines/United States Marine Corps - Marine
• United States Coast Guard - Coast Guardsman
• United States National Guard - National Guardsman

Note that within the British Armed Forces, these specific terms are often used only to refer to non-commissioned personnel (an army officer may speak of "my soldiers", for instance).

Scouts

In the British Boy Scouts and Boy Scouts of America, a troop is an organized local group that meets regularly.