prin·ci·pal 
(prĭnsə-pəl)
(prĭnsə-pəl)adj.
1. First or highest in rank or importance. See Synonyms at chief.
2. Of, relating to, or being financial principal, or a principal in a financial transaction.
n.
1.
a. One who holds a position of presiding rank, especially the head of an elementary school, middle school, or high school.
b. A main participant in a situation, especially a financial transaction.
c. A person having a leading or starring role in a performance, such as the first player in a section of an orchestra.
2.
a. An amount of capital originally borrowed or invested, as opposed to the interest paid or accruing on it.
b. The most significant part of an estate, as opposed to minor or incidental components.
3. Law
a. The person on behalf of whom an agent acts.
b. The person having prime responsibility for an obligation as distinguished from one who acts as surety or as an endorser.
c. The main actor in the perpetration of a crime.
4. Architecture Either of a pair of inclined timbers forming the sides of a triangular truss for a pitched roof.
[Middle English, from Old French, from Latin prīncipālis, from prīnceps, prīncip-, leader, emperor; see per1 in the Appendix of Indo-European roots.]
princi·pal·ly adv.
princi·pal·ship′ n.
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.