Principal (adjective or noun) conveys the meaning of "primary" or "chief" - like [as adjective] e.g. the principal aim of a series of tasks,or [as noun] the principal of a school. It is also used as the chief part of a loan (i.e., not the interest), as in apply the extra $50 in this month's mortgage payment toward the principal. synonyms: Chief, main, premier, leading
Principle (noun [only!]) is a rule, law or general truth. The principles of mathematics. synonyms: Axiom, Precept, Doctrine, Golden rule, Moral