Mercenary
French Foreign Legionnaires serve in the French Foreign Legion, which deploys and fights as an organized unit of the French Army. This means that as members of the armed forces of Britain, India, and France these soldiers are not classed as mercenary soldiers per APGC77 Art 47.e and 47.f. Volunteers for the Ukraine Foreign legion have three-year contracts, and are eligible for Ukrainian citizenship (the probation period being the duration of the war).

Private military companies: The private military company (PMC) is a private company providing armed combat or security services for financial gain. PMCs refer to their personnel as security contractors or private military contractors. PMC contractors are civilians (in governmental, international, and civil organizations) authorized to accompany an army to the field; thus, the term civilian contractor. PMCs may use armed force, defined as: "legally established enterprises that make a profit, by either providing services involving the potential exercise of [armed] force in a systematic way and by military means, and/or by the transfer of that potential to clients through training and other practices, such as logistics support, equipment procurement, and intelligence gathering".
Private paramilitary forces are functionally mercenary armies, though they may serve as security guards or military advisors; however, national governments reserve the right to control the number, nature, and armaments of such private armies, arguing that, provided they are not pro-actively employed in front-line combat, they are not mercenaries. In February 2002, a British Foreign and Commonwealth Office (FCO) report about PMCs noted that the demands of the military service from the UN and international civil organizations might mean that it is cheaper to pay PMCs than use soldiers. PMC "civilian contractors" tend to have poor reputations among professional government soldiers and officers—the U.S. Military Command has questioned their war zone behavior.
In September 2005, Brigadier General Karl Horst, deputy commander of the Third Infantry Division charged with Baghdad security after the 2003 invasion of Iraq, said of DynCorp and other PMCs:

These guys run loose in this country and do stupid stuff. There's no authority over them, so you can't come down on them hard when they escalate force... They shoot people, and someone else has to deal with the aftermath. It happens all over the place.
In 2004, the US and Coalition governments hired PMCs for security in Iraq. In March 2004, four Blackwater employees escorting food supplies and other equipment were attacked and killed in Fallujah in a videotaped attack; the killings and subsequent dismemberments were a cause for the First Battle of Fallujah.

Afghan war operations also boosted the business. The United States has made extensive use of PMCs in Afghanistan since 2001, mostly in a defensive role. PMC teams have been used to guard bases and to protect VIPs from Taliban assassins, but almost never in offensive operations. One mercenary stated about his work in Afghanistan: "We are there purely to protect the principals and get them out, we're not there to get into huge firefights with the bad guys". One team from DynCorp provided bodyguards for President Hamid Karzai.

Colombia: In 2006, a US congressional report listed a number of PMCs and other enterprises that have signed contracts to carry out anti-narcotics operations and related activities as part of Plan Colombia. Referring to the use of American PMCs in Colombia, the former US Ambassador to Colombia Myles Frechette has said: "Congress and the American people don't want any servicemen killed overseas. So it makes sense that if contractors want to risk their lives, they get the job".
Not only have foreign PMCs worked in Colombia, but a disproportionate number of the mercenaries with PMCs are Colombian, as Colombia's long history of civil war has led to a surplus of experienced soldiers. Also, Colombian soldiers are much cheaper than soldiers from developed countries. PMCs from several Middle Eastern countries have signed contracts with the Colombian Defense Ministry to carry out security or military activities.

UN concerns over legality of PMCs: The United Nations questions whether PMC soldiers are sufficiently accountable for their war zone actions. A common argument for using PMCs (used by the PMCs themselves), is that PMCs may be able to help combat genocide and civilian slaughter where the UN or other countries are unwilling or unable to intervene. Yet, after considering using PMCs to support UN operations, Kofi Annan, the Secretary-General of the United Nations, decided against it.
In October 2007, the United Nations released a two-year study that stated, that although hired as "security guards", private contractors were performing military duties. The report found that the use of contractors such as Blackwater was a "new form of mercenary activity" and illegal under international law.
Most countries, including the United States and the United Kingdom, are not signatories to the 1989 United Nations Mercenary Convention banning the use of mercenaries. A spokesman for the U.S. Mission to U.N. denied that Blackwater security guards were mercenaries, saying "Accusations that U.S. government-contracted security guards, of whatever nationality, are mercenaries is inaccurate and demeaning to men and women who put their lives on the line to protect people and facilities every day."

History: 

Europe: 

Classical era: 

Greek mercenaries in Persian Empire: Xerxes I, King of Persia, employed Arcadian mercenaries during his invasion of Greece.
In Anabasis, Xenophon recounts how Cyrus the Younger hired a large army of Greek mercenaries (the "Ten Thousand") in 401 BC to seize the throne of Persia from his brother, Artaxerxes II. Though Cyrus' army was victorious at the Battle of Cunaxa, Cyrus himself was killed in battle and the expedition rendered moot. Stranded deep in enemy territory, the Spartan general Clearchus and most of the other Greek generals were subsequently killed by treachery. Xenophon played an instrumental role in encouraging "The Ten Thousand" Greek army to march north to the Black Sea in an epic fighting retreat.
The Sileraioi were a group of ancient mercenaries most likely employed by the tyrant Dionysius I of Syracuse.
In 378 BC the Persian Empire hired the Athenian general Iphicrates with his mercenaries in the Egyptian campaign.
The Mania, who was a sub-satrap, used Greek mercenaries in order to capture other cities in the region.
Memnon of Rhodes (380–333 BC) was the commander of the Greek mercenaries working for the Persian King Darius III when Alexander the Great of Macedonia invaded Persia in 334 BC and won the Battle of the Granicus River. Alexander also employed Greek mercenaries during his campaigns. These were men who fought for him directly and not those who fought in city-state units attached to his army.

Greek mercenaries in ancient India: Tamil poems described the Greek soldiers who served as mercenaries for Indian kings as: "The valiant-eyed Yavanas (Greeks), whose bodies were strong and of terrible aspect".
Alfred Charles Auguste Foucher said that some of the troops of Mara in the Gandhara sculptures may represent Greek mercenaries.
Stephanus of Byzantium wrote about a city called Daedala or Daidala (Ancient Greek: Δαίδαλα) in India, which he called Indo-Cretan, most probably because it was a settlement of Cretan mercenaries.

Carthage: Carthage contracted Balearic Islands shepherds as slingers during the Punic Wars against Rome. The vast majority of the Carthaginian military – except the highest officers, the navy, and the home guard – were mercenaries.
Xanthippus of Carthage was a Spartan mercenary general employed by Carthage.
Greek mercenaries were hired by Carthage to fight against the Dionysius I of Syracuse. Dionysius made Carthage pay a very high ransom for the Carthaginian prisoners, but he left the Greek mercenaries prisoners free without any ransom. This made the Carthaginians suspicious of their Greek mercenaries and discharged them all from their service. With this trick Dionysius did not have to fight again against the Greek mercenaries of Carthage who were very dangerous enemies.

Byzantine Empire: In the late Roman Empire, it became increasingly difficult for Emperors and generals to raise military units from the citizenry for various reasons: lack of manpower, lack of time available for training, lack of materials, and, inevitably, political considerations. Therefore, beginning in the late 4th century, the empire often contracted whole bands of barbarians either within the legions or as autonomous foederati. The barbarians were Romanized and surviving veterans were established in areas requiring population. The Varangian Guard of the Byzantine Empire is the best known formation made up of barbarian mercenaries (see next section).

Other: Members of independent Thracian tribes such as the Bessi and Dii often joined the ranks of large organized armies as mercenaries.
The Sons of Mars were Italian mercenaries used by the Greek kings of Syracuse until after the Punic Wars.
A figure in oral legend, Milesius was given the princess Scota after conducting a successful campaign for Ancient Egypt.
Mithridates VI Eupator recruited a large number of Iranians along with the Galatians into the Pontic army during the Mithridatic Wars against Rome, using the Leucosyri, Persians and Scythians.
Illyrians were hired across the Balkans and further. They were known for their unreliability.