1) In commercial law, the total amount of a loan, not including any capitalized fees or interest. 2) When creating a power of attorney or other legal document, the person who appoints an attorney-in-fact or agent to act on his or her behalf. 3) In the law of trusts, the property of the trust, as opposed to the income generated by that property. The principal is also known as the trust corpus (that's Latin for "body"). 4) In criminal law, the main perpetrator of a crime.