Definitions 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

  Example: "She sent unruly pupils to see the principal"

• noun:   capital as contrasted with the income derived from it
• noun:   the original amount of a debt on which interest is calculated
• noun:   an actor who plays a principal role