What is Principal ?
Principal is (adj) most important The country’s principal products are paper and wood. She played a principal role in setting up the organisation. Synonym main (noun) 1. the head of a school or college The principal wants to see you in her office. 2. a main performer in a theatrical performance The principals were quite good but the chorus was awful. 3. money on which interest is paid, capital which has been invested Up to now you’ve been paying interest, but now you can start repaying some of the principal. (NOTE: Do not confuse with principle.)
source: Easier English, Student Dictionary Upper Intermediate Level