Answer:
Mercenary / Private Military Companies (PMCs)
The term mercenary is applied to a variety of historical situations which do not appear to have elements in common. Casca, the eternal mercenary, pulled the duty of nailing Christ to the Cross and was doomed to spend eternity as a soldier, a career that can lead to billets like sitting on five-gallon water cans in the cold desert wind on Christmas Eve in Saudi Arabia.
The recruitment and use of mercenaries is legally defined in international law, in the 1977 Additional Protocol I to the 1949 Geneva Conventions, as well as the 1989 International Convention against the Recruitment, Use, Financing and Training of Mercenaries, to which Ukraine is a party. The criteria for a mercenary are taking part directly in hostilities, motivated primarily by the desire for private gain, being paid substantially in excess of that promised or paid to combatants of similar ranks and functions in the armed forces of the party to the conflict, and not being a national of the party to the conflict or a resident of the territory controlled by a party to the conflict. The recruitment and use of mercenaries is prohibited by international law.
With regard to foreign fighters, there is no internationally agreed legal definition of foreign fighters, nor a specific regime governing them. A foreign fighter is generally understood to refer to individuals who leave their country of origin or habitual residence and become involved in violence as part of an insurgency or non-State armed group in an armed conflict. Foreign fighters are motivated by a range of factors, notably ideology, but can also be attracted fight for financial reward. Foreign fighters are obliged to respect applicable rules of international human rights and humanitarian law during armed conflicts.
In Deccember 1989 the UN General Assembly adopted the "International Convention against the Recruitment, Use, Financing and Training of Mercenaries", which noted that "the recruitment, use, financing and training of mercenaries should be considered as offences of grave concern to all States and that any person committing any of these offences should either be prosecuted or extradited..." This convention defined a mercenary, as a person, who among other attributes "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 rank and functions in the armed forces of that party..."
