Definition of "Principal" :

noun: the major party to a financial transaction at a stock exchange; buys and sells for his own account

noun: the educator who has executive authority for a school

"She sent unruly pupils to see the principal."

noun: (criminal law) any person involved in a criminal offense, regardless of whether the person profits from such involvement

noun: an actor who plays a principal role

noun: capital as contrasted with the income derived from it

noun: the original amount of a debt on which interest is calculated

adjective: most important element

"The principal rivers of America."