principal

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective First or highest in rank or importance. synonym: chief.
• adjective Of, relating to, or being financial principal, or a principal in a financial transaction.
• noun One who holds a position of presiding rank, especially the head of an elementary school, middle school, or high school.
• noun A main participant in a situation, especially a financial transaction.
• noun A person having a leading or starring role in a performance, such as the first player in a section of an orchestra.
• noun An amount of capital originally borrowed or invested, as opposed to the interest paid or accruing on it.
• noun The most significant part of an estate, as opposed to minor or incidental components.
• noun The person on behalf of whom an agent acts.
• noun The person having prime responsibility for an obligation as distinguished from one who acts as surety or as an endorser.
• noun The main actor in the perpetration of a crime.
• noun Architecture Either of a pair of inclined timbers forming the sides of a triangular truss for a pitched roof.

from The Century Dictionary.

• noun A main truss, as of a roof, where there may be many principals.
• noun In the Philippine Islands, every member, present or past, of the council of a pueblo; also, a first-born son of a gobernadorcillo or of a cabeza de barangay. See barangay.
• Chief; highest in rank, authority, value, or importance; most considerable; main; first: as, the principal officers of a government; the principal points in an argument; the principal products of a country.
• Of or pertaining to a prince; princely.
• Synonyms Leading, great, capital, cardinal, supreme.
• noun A chief or head; one who takes a leading part; one primarily concerned in an action, and not an auxiliary, accessory, assistant, or agent: as, the principals in a duel.
• noun A governor or presiding officer; one who is Chief in authority. ; ; ;
• noun In law: A person who, being sui juris, and competent to do an act on his own account, employs another person to do it; the person from whom an agent's authority is derived. Compare master, 2.
• noun A person for whom another becomes surety; one who is liable for a debt in the first instance.
• noun In testamentary and administration law, the corpus or capital of the estate, in contradistinction to the income.
• noun In criminal law, the actor in the commission of a crime; a person concerned in the commission of a crime, whether he directly commits the act constituting the offense or instigates or aids and abets in its commission.
• noun In com., money bearing interest; a capital sum lent on interest, due as a debt or used as a fund: so called in distinction to interest or profits.
• noun In organ-building, a stop of the open diapason group, usually giving tones an octave above the pitch of the digitals used, like the octave.
• noun A musical instrument used in old orchestral music, especially that of Handel — a variety of trumpet, probably having a larger tube than the ordinary tromba.
• noun In music: The subject of a fugue: opposed to answer.
• noun A soloist or other leading performer.
• noun Same as principal rafter. See rafter.
• noun In the fine arts, the chief motive in a work of art, to which the rest are to be subordinate; also, an original painting or other work of art.
• noun One of the turrets or pinnacles of waxwork and tapers with which the posts and center of a hearse were formerly crowned.
• noun An important personal belonging; an heirloom.
• noun In ornithology, one of the primaries.

from the GNU version of the Collaborative International Dictionary of English.

• adjective Highest in rank, authority, character, importance, or degree; most considerable or important; chief; main
• adjective A Latinism, obsolete Of or pertaining to a prince; princely.
• adjective See Axis of a curve, under Axis.
• adjective (Geom.) three lines in which the principal planes of the solid intersect two and two, as in an ellipsoid.
• adjective (Law) See under Challenge.
• adjective See Plane of projection (a), under Plane.
• adjective (Geom.) three planes each of which is at right angles to the other two, and bisects all chords of the quadric perpendicular to the plane, as in an ellipsoid.
• adjective (Persp.) the projection of the point of sight upon the plane of projection.
• adjective (Persp.) the line drawn through the point of sight perpendicular to the perspective plane.
• adjective (Crystallog.) a plane passing through the optical axis of a crystal.
• noun A leader, chief, or head; one who takes the lead; one who acts independently, or who has controlling authority or influence; ; -- distinguished from a subordinate, abettor, auxiliary, or assistant.
• noun The chief actor in a crime, or an abettor who is present at it, -- as distinguished from an accessory.
• noun A chief obligor, promisor, or debtor, -- as distinguished from a surety.
• noun One who employs another to act for him, -- as distinguished from an agent.