What exactly is a mercenary? My belief was that it was a soldier/fighter/warrior fighting in a political conflict with a personal interest (such as money to be gained). But many video games and random stories portrait them as random soldiers who fight in any conflict as long as the pay is good; political affiliation or not. Basically a jack of all trades sort of person. If I’m not making a lot of sense it’s because I’m confused myself. Sorry and thanks in advance!
A mercenary is just a soldier for hire. Usually this was entire companies of soldiers who were hired as a unit, but the basic idea is there. The term itself is pejorative, and gets applied in a wider range of circumstances as an insult. Someone who acts for money without regard to their own loyalty or ethics may be described as being mercenary. (The word itself can be used as either a noun or adjective depending on context, though the general meaning remains the same.)
Historically, mercenaries tended to be better trained than conventional standing forces. The thought process here is that maintaining a standing army in medieval Europe was fairly expensive, so you’d maintain a small force (if any), and then press or draft peasant infantry into service when the time came. Within this context, a mercenary company, who’d accumulated years of combat experience would be a significantly more effective force.
Mercenaries could have a unified national identity, and in some cases may even be hired out by their government directly, or they could be an ad hoc band of soldiers, gathered indiscriminately in their travels.
Under international law, there are a few wrinkles to defining when someone is, legally, a mercenary. They need to be hired by a nation to fight for it, and they cannot be from that nation. This only becomes relevant when dealing with situations like war crimes, or treatment of prisoners. This means that private soldiers hired by a corporation aren’t technically mercenaries under the legal definition, even though they’re still called that. This also means when a nation hires private soldiers from their own population, those soldiers aren’t, legally, mercenaries. There’s a pretty solid argument that domestic PMCs (Private Military Companies) should be legally classified as mercenaries, but the practice’s rise is very recent.
I mentioned the term is pejorative, this is in large part because mercenaries fought for coin, rather than out of patriotic duty or loyalty. As a result they’re viewed as dishonorable and untrustworthy. There may be some basis to this, but it’s also why the term has a more generalized meaning. Someone who puts their pay above their principles may be described as mercenary. For example, a political operative with no loyalty to their ideological beliefs could be described as “mercenary,” even though that’s clearly not the traditional meaning of the term. Another possible example would be a character who would willingly sell out their friends for a bounty. Again, not a soldier for hire, but simply amoral behavior in pursuit of cash.