Definition: [n] the major party to a financial transaction at a stock exchange; buys and sells for his own account
[n] the educator who has executive authority for a school; "she sent unruly pupils to see the principal"
[n] an actor who plays a principal role
[n] capital as contrasted with the income derived from it
[n] the original amount of a debt on which interest is calculated

http://www.hyperdictionary.com/dictionary/principal