principle v. principal

A principle is a fundamental truth, doctrine, rule of conduct, natural phenomenon, or quality.

As an adjective, principal means first, chief, or foremost. As a noun, a principal is:

• the head of a school
• a person with a leading or starring role
• the original amount of a debt, on which interest is calculated
• a person who empowers an agent to act as a representative
• someone directly responsible for committing a crime
• the main body of an estate, as distinguished from its interest or revenue

Grammar Girl on principle v. principal