principal, principle

Principal is an adjective meaning “chief” or “leading.”

• The principal element in the group’s strategy was surprise.
• Lorne Kennedy played the principal character in the comedy Present Laughter.

Principal is also a noun indicating a person in authority or a sum of money, usually a loan.

• The high school principal supported the policy against bullying.
• The monthly mortgage payment includes principal, interest and taxes.

Principle, which can be used only as a noun, refers to a universal law or rule of conduct.

• According to the uncertainty principle, one alters subatomic particles simply by observing them.
• Hilary has very high principles; she never lies, steals or gossips.