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"
(Nearly any one can become a mercenary but they are often an ex-member of a faction's military forces as military training is often required for the jobs they take)