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" (Additional Protocol I to the Geneva Convention of August 1949). A non-conscript professional member of a regular army is not considered to be a mercenary although he gets monetary reward from his service.

As a result of the assumption that a mercenary is essentially motivated by money, the term "mercenary" usually carries negative connotations, though that can be a compliment in some contexts. 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 are not mercenaries under the laws of war, since although they may meet many of the requirements of Article 47 of the 1949 Additional Protocol I, they are exempt under clauses 47(a)(c)(d)(e)&(f); some journalists describe them as mercenaries regardless.