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. Fundamental stop on a pipe organ. Good pipe organs usually have several principals.