principal

Concise Oxford English Dictionary © 2008 Oxford University Press:
principal/ˈprɪnsəpl/

▶adjective

• 1 first in order of importance; main.

• 2 denoting an original sum of money invested or lent.

▶noun

• 1 the most important or senior person in an organization or group.

  ■ the head of a school or college.

  ■ (in certain professions) a fully qualified practitioner.

• 2 a sum of money lent or invested, on which interest is paid.

• 3 a person for whom another acts as an agent or representative.

  ■ Law a person directly responsible for a crime.

• 4 a main rafter supporting purlins.

• 5 an organ stop sounding a set of pipes typically an octave above the diapason.

– derivatives
principalship noun.

– origin ME: via OFr. from L. principalis ‘first, original’, from princeps, princip- ‘first, chief’.

usage: On the confusion of principal and principle, see usage at principle.