Principal

A principal is:

1. The head of an institution. See Dean or School principal.
2. A natural person or legal entity acting on its/his/her own behalf, not as an agent or surety.
3. In finance, the original amount of a debt on which interest is calculated. Also the amount still owed on a debt which has been partially paid, sometime also called remaining principal or outstanding balance.
4. In Access Control, an entity with some authentication credentials. This is distinct from groups which are collections of principals (and other groups).
5. Fundamental stop on a pipe organ. Good pipe organs usually have several principals.