Obviously, you can't just leave the "mercenary" definition hanging on its own. There are other relevant definitions alongside it for different types of forces (ones that come to mind):
1. Professional soldiers.
2. Volunteers.
3. Conscripts/draftees
4. Militia
in addition to
5. Mercenaries
6. Irregulars/guerrilla's

These classifications are not necessarily mutually exclusive. Each has its own definition and connotation though.