What makes a mercenary?

[Somebody Else]

I see it that a soldier has sworn an oath to obey his (or indeed her) ruler, so the Queen and her appointed government for instance. (Trying vaguely to remember my attestation).
A mercenary, on the other hand, swears no such oath, and is only bound by such reward as he or she gains from fighting. A mercenary can walk away and only lose his pay, a soldier would lose his honour (and probably be shot for desertion too).