A mercenary is one who fights for money. Armed groups of trained soldiers who would hire themselves out were common in more ancient times. Mercenaries could be either individuals or groups of soldiers. As examples, Greek mercenaries were popular in ancient times, fighting in various battles for the Persian Empire while the Swiss made sought after mercenaries around the 1400s for the various battles in Europe. The Flying Tigers were mercenary pilots who fought for China in the Second Sino-Japanese War and were paid a bonus for every Japanese plane shot down.
Mercenaries in Fiction
The concept of Mercenaries play a major role in the Metal Gear Solid series.