There is a legal definition in the additional protocol of the geneva convention

Article 47.-Mercenaries

1. A mercenary shall not have the right to be a combatant or a prisoner of war.

2. A mercenary is any person who:

( a ) Is specially recruited locally or abroad in order to fight in an armed conflict;

( b ) Does, in fact, take a direct part in the hostilities;

( c ) 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;

( d ) Is neither a national of a Party to the conflict nor a resident of territory controlled by a Party to the conflict;

( e ) Is not a member of the armed forces of a Party to the conflict; and

( f ) Has not been sent by a State which is not a Party to the conflict on official duty as a member of its armed forces.
http://www.ohchr.org/english/law/protocol1.htm