A troop is a military unit, originally a small force of cavalry, subordinate to a squadron and headed by the troop leader. Cavalry soldiers of private rank are called troopers (abbreviated Tpr., not troupers). Today, troop has different specific meanings in different armed forces.

Generally, soldiers may be referred to as troops.

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.

U.S. Cavalry usage

In the United States Army, in the cavalry branch, a troop is the equivalent unit to the infantry company, commanded by a captain and consisting of 3 or 4 platoons, and subordinate to a squadron or battalion.

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. 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.