1. First, highest, or foremost in importance, rank, worth, or degree; chief. See Synonyms at chief.
2. Of, relating to, or being financial principal, or a principal in a financial transaction.

1. One who holds a position of presiding rank, especially the head of an elementary school or high school.
2. A main participant in a situation.
3. A person having a leading or starring role.
4. 1. The capital or main body of an estate or financial holding as distinguished from the interest or revenue from it.
   2. A sum of money owed as a debt, upon which interest is calculated.
   3. A person who empowers another to act as his or her representative.
   4. The person having prime responsibility for an obligation as distinguished from one who acts as surety or as an endorser.
   5. One who commits or is an accomplice to a crime.
5. Law
   1. A person who empowers another to act as his or her representative.
   2. The person having prime responsibility for an obligation as distinguished from one who acts as surety or as an endorser.
   3. One who commits or is an accomplice to a crime.
6. Architecture Either of a pair of inclined timbers forming the sides of a triangular truss for a pitched roof.

Usage Note: Principal and principle are often confused but have no meanings in common. Principle is only a noun and usually refers to a rule or standard. Principal is both a noun and an adjective. As a noun, it has specialized meanings in law and finance, but in general usage it refers to a person who holds a high position or plays an important role: a meeting among all the principals in the transaction. As an adjective it has the sense of "chief" or "leading": The coach's principal concern is the quarterback's health.