Merriam-Webster Collegiate Dictionary - principal

 I. adjective  Etymology: Middle English, from Anglo-French, from Latin ~is, from princip-, princeps  Date: 14th century  1. most important, consequential, or influential ; chief the ~ ingredient the region's ~ city  2. of, relating to, or constituting ~ or a ~  Usage: see principle  • ~ly adverb  II. noun  Date: 14th century  1. a person who has controlling authority or is in a leading position: as  a. a chief or head man or woman  b. the chief executive officer of an educational institution  c. one who engages another to act as an agent subject to general control and instruction; specifically the person from whom an agent's authority derives  d. the chief or an actual participant in a crime  e. the person primarily or ultimately liable on a legal obligation  f. a leading performer ; star  2. a matter or thing of primary importance: as  a.  (1) a capital sum earning interest, due as a debt, or used as a fund  (2) the corpus of an estate, portion, devise, or bequest  b. the construction that gives shape and strength to a roof and is usually one of several trusses; broadly the most important member of a piece of framing  Usage: see principle  • ~ship noun