Mercenary
Historically, a mercenary is a professional soldier for hire, serving whatever lord or army decides to pay them. Many examples existed in period - The Free Companies, the Italian condottieri, John Hawkwood and the White Company, Roger de Flo and the Catalan Grand Company, the Swiss Guard, the Varangian Guard - and these are but a few examples.
In the SCA, a mercenary is a person or group that offers themselves out as fighters for pay, and typically don't have allegiance to a particular barony/local group - possibly even hiring out to other kingdoms. Rather than hiring out for money, the typical pay for a mercenary household may vary. Meals, dance performances, coin of the realm (cookies), beer or other drink, baubles, and even poems have all been used to hire SCA mercenaries. Like most other fighters, their main desire is to fight in wars for the joy of it - the pay is a bonus, and is not necessarily required.
Also paralleling history, complications can arise when members of a mercenary group or household are in some form of fealty - knights, champions, personal fealty, and so forth. This may cause individuals to end up on the opposite side of their company.
Another parallel is that some hold a negative view of mercenaries in the SCA. The SCA is a volunteer driven organization, and the idea of doing something "for pay", may rankle. Additionally, many gentles are deeply involved, and loyal, to their local group (shire, barony, etc.), and may feel those without such loyalties are problematic. This can be somewhat self-reinforcing, as many who are not very involved with a local group (or have had poor experiences with one) may find mercenary groups an attractive alternative. This may be because they offer an alternative home to a local group, including rewards and recognition (pay). Many mercenary groups or individuals are very involved with their local group, or eventually become so.