A principle is a firm belief or conviction that one has about something, or a rule or law about the way something works: It would be against my principles to do such a thing; A thermometer works on the principle that mercury expands when warm and contracts when cool. A principal is the head of a college: She became Principal in 1991. Principle is always a noun, but principal can also be an adjective meaning 'main', or 'chief': My principal objection is the cost.