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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.