A Private Military Company (PMC) or (private military or security companies) provides military and security services. These combatants are commonly known as mercenaries, though modern-day PMCs refer to their staff as security contractors, private military contractors or private security contractors, and refer to themselves as private military corporations, private military firms, private security providers or military service providers. Private military companies refer to their business generally as the private military industry or The Circuit, in an attempt to avoid the stigma often associated with mercenaries.^{[citation needed]} The hiring of mercenaries is a common practice in the history of armed conflict.

The services and expertise offered by PMCs are typically similar to those of governmental military or police forces, most often on a smaller scale. While PMCs often provide services to train or supplement official armed forces in service of governments, they can also be employed by private companies to provide bodyguards for key staff or protection of company premises, especially in hostile territories. However, contractors who use offensive force in a war zone could be considered unlawful combatants, in reference to a concept outlined in the Geneva Conventions and explicitly specified by the US Military Commissions Act.

A company is a military unit, typically consisting of 80–225 soldiers and usually commanded by a Captain, Major or Commandant. Most companies are formed of three to five platoons although the exact number may vary by country, unit type, and structure. Several companies are grouped to form a battalion or regiment, the latter of which is sometimes formed by several battalions.

Rifle companies consist of four platoons and a company headquarters. The artillery equivalent of a company is a battery and within armoured and engineering units, the equivalent is known as a squadron.

The British Army infantry normally identifies its rifle companies by letter (usually, but not always, A, B and C) within a battalion, usually with the addition of a headquarters company and a support/heavy weapons company. Some units name their companies after regimental battle honours; this is commonly the case for composite units, for example the London Regiment with its Somme, Messines and Cambrai companies. The foot guards regiments use traditional names for some of their companies, for example Queen's Company, Left Flank, Prince of Wales's Company etc.

A mercenary is a person who takes part in an armed conflict, who is not a national or a party to the conflict, and is "motivated to take part in the hostilities essentially by the desire for private gain and, in fact, is promised, by or on behalf of a party to the conflict, material compensation substantially in excess of that promised or paid to combatants of similar ranks and functions in the armed forces of that Party".

As a result of the assumption that a mercenary is essentially motivated by money, the term mercenary usually carries negative connotations. There is a blur in the distinction between a mercenary and a foreign volunteer, when the primary motive of a soldier in a foreign army is uncertain. For instance, the French Foreign Legion and the Gurkhas of the British and Indian armies are not mercenaries under the laws of war, since although they may meet many of the requirements of Article 47 of Protocol I to the Geneva Conventions, they are exempt under clauses 47(a)(c)(d)(e)&(f); some journalists describe them as mercenaries nevertheless.