Principal - principle

These two homophones form one of the sets of homophones listed by the then Poet Laureate Robert Bridges.
(For more, see Bridges homophones). AWE has a category listing our articles on each of these..

Don't confuse principal and principle.

• A principle (always a noun) is an underlying idea, or a moral belief, etc.
  
• A principal is either
  • a noun meaning a head, a senior person in an organization or unit (e.g. the Head of a school or the Head of Maths within it); or
  • an adjective describing the most important thing, e.g. 'The principal point in this lecture is ....'.