Note: these are only a few terms that we'll be using frequently on here. There are more out there.
Plural: an umbrella term for more than one person sharing a physical body.
Headmate/System-mate: a member of a system.
Fronting: controlling the body.
Co-fronting: Same as fronting, but with another person.
Tulpa: a headmate/thoughtform that has been intentionally created.
