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Atroop is amilitarysub-subunit, originally a small formation ofcavalry, subordinate to asquadron. In many armies a troop is the equivalent element to theinfantrysection orplatoon. Exceptions are theUS Cavalry and theKing's Troop Royal Horse Artillery where a troop is a subunit comparable to an infantrycompany orartillery battery. Historically the remainder of theRoyal Horse Artillery used the termtroop in the same manner but they eventually aligned with the rest of theRoyal Regiment of Artillery in referring to troops as subordinate to artillery batteries.
Troops is often used to refer to the other members of one's company or cause, but because of its military connotations, it conveys a particularly altruistic type of dedicated worker. Traditionally,troops refers to the soldiers in a military.
A cavalry soldier ofprivate rank is called a "trooper" in manyCommonwealth armies (abbreviated "Tpr", not to be confused with "trouper").
A related sense of the term,troops refers to members of the military collectively, as in "the troops"; seeTroop (disambiguation).
Today, a troop is defined differently in different armed forces.
In theAustralian Army a troop is the equivalent of a platoon sized element in units of certain corps, those being:[1]
The SASR is the only unit in theRoyal Australian Infantry Corps to use the termtroop to refer to its platoon-size elements. SASR troops are also unusual as they are commanded by a captain—most troop or platoon-sized elements are commanded by a lieutenant. In most cases, units which refer to platoon sized elements as troops refer to company-sized elements as squadrons and battalion-sized elements as regiments (regiments in theRAA use the term 'battery' for company-sized elements). Privates in the Royal Australian Armoured Corps and SASR hold the rank "trooper", however this is not the case for any other corps or units, which use the termtroops.[2]

In theBritish Army the definition of a troop varies bycorps.
Other Army corps do not use the term.
In theRoyal Marines, a troop is the equivalent to an Army platoon; a carryover from the organisation of theBritish Commandos in World War II.
In theCanadian 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 theUnited 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 are called a troop within a regiment. Companies were renamed troops in 1883.[3] In some instances, an infantry company may be titled as a "troop" due to its presence in a cavalry squadron; this is typically the case for thedismounted reconnaissance troop (DRT) of aRSTA squadron, in which an infantry company-sized element is part of a cavalry-branched squadron.[4]
In the United States,state police forces are often regionally divided into troops. This usage came from these organizations modelling themselves on the US Army, and especially theolder cavalry units. For this same reason the state police and highway patrol personnel of most states are known as "troopers" rather than "officers".
InScouting, ascout troop is a unit made up of scouts orguides from the same locality under aleader. In the case of Guides, the term "company" is used more often, and was used bythe founder in his first books about guiding.