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A troop is a military unit, originally a small force of cavalry, subordinate to a squadron and headed by the troop leader. In many armies a troop is the equivalent unit to the infantry section or platoon. Exceptions are the Royal Horse Artillery and the US Cavalry, where troop refers to an infantry company or artillery battery.
A related sense of the term troop refers to soldiers collectively, as in the troops; see Troop (disambiguation).
Troops in various forces 
Today, a troop is defined differently in different armed forces.
- Royal Australian Armoured Corps
- Royal Australian Engineers
- Royal Australian Corps of Signals
- Australian Army Aviation
- Royal Australian Corps of Transport
- Royal Australian Survey Corps (now disbanded)
- Special Air Service Regiment (SASR)
The SASR is the only unit in the Royal Australian Infantry Corps to use the term troop to refer to its platoon size elements. SASR troops are also unusual as they are commanded by a captain—most troop/platoon sized elements are commanded by a lieutenant. In all cases, units which refer to platoon sized elements as troops refer to company-sized elements as squadrons and battalion-sized elements as regiments. Privates in the RAAC and SASR hold the rank "trooper", however this is not the case for any other Corps/units which use the term troops.
- Household Cavalry and Royal Armoured Corps: Three or four armoured fighting vehicles commanded by a subaltern, i.e. effectively the same level element as an infantry platoon. A unit of two to four guns or launchers, or an equivalent headquarters unit. In the Royal Horse Artillery, a troop used to be the equivalent to a battery in other artillery units.
- Royal Engineers, Royal Corps of Signals, Royal Logistic Corps, Special Air Service and Honourable Artillery Company: A unit equivalent in size to a platoon in other corps, divided into sections or Patrols.
Other army corps do not use the term.
In the Royal Marines, a troop is the equivalent to an army platoon.
In the Canadian Army, a troop is the equivalent of a platoon within the Armoured, Artillery, Engineer, and Signals branches. Two to four troops comprise the main elements of a squadron.
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 three or four platoons, and subordinate to a squadron (battalion). Companies were renamed troops in 1883.
Troops in civilian organizations 
In the United States, state police forces are often regionally divided into troops. This usage came about from these organizations modeling themselves off the US Army, and especially the older cavalry units. For this same reason the state police and highway patrol personnel of most states are known as "trooper" rather than "officer".
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