Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 108. Mercenaries
Section A. Definition of mercenaries
The UK LOAC Pamphlet (1981) states: “A mercenary is a person who takes part in the conflict for private gain, who is not a member of any organised armed forces and has no connection with the countries involved in the conflict.” 
United Kingdom, The Law of Armed Conflict, D/DAT/13/35/66, Army Code 71130 (Revised 1981), Ministry of Defence, prepared under the Direction of The Chief of the General Staff, 1981, Section 3, p. 10, § 7.
The UK LOAC Manual (2004) states:
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. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 4.10.1.