Noun

• S: (n) principal (the original amount of a debt on which interest is calculated)
• S: (n) principal, school principal, head teacher, head (the educator who has executive authority for a school) "she sent unruly pupils to see the principal"
• S: (n) star, principal, lead (an actor who plays a principal role)
• S: (n) principal, corpus, principal sum (capital as contrasted with the income derived from it)
• S: (n) principal ((criminal law) any person involved in a criminal offense, regardless of whether the person profits from such involvement)
• S: (n) principal, dealer (the major party to a financial transaction at a stock exchange; buys and sells for his own account)

Adjective

• S: (adj) chief, main, primary, principal, master (most important element) "the chief aim of living"; "the main doors were of solid glass"; "the principal rivers of America"; "the principal example"; "policemen were primary targets"; "the master bedroom"; "a master switch"