A troop is a military sub-subunit, originally a small formation of cavalry, subordinate to a squadron. In many armies a troop is the equivalent element to the infantry section or platoon. Exceptions are the US Cavalry and the King's Troop Royal Horse Artillery where a troop is a subunit comparable to an infantry company or artillery battery. Historically the remainder of the Royal Horse Artillery used the term troop in the same manner but they eventually aligned with the rest of the Royal 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 of private rank is called a "trooper" in many Commonwealth 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"; see Troop (disambiguation).

Today, a troop is defined differently in different armed forces.

In the Australian Army a troop is the equivalent of a platoon sized element in units of certain corps, those being:

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 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 the RAA 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 term troops.

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