In general, a person or thing that is more important than others when identified for a particular purpose. More specifically, in various contexts:
1. Someone who authorizes another to act in their place. See Agent, Fiduciary, and Fiduciary duty.
2. The basic amount of a debt or investment - which excludes any interest, profits, or other additional earnings on the basic underlying amount.
3. Someone with primary responsibility to perform an obligation. See Surety.
4. The corpus of a trust, which is composed of the assets funding the trust.