Principal means "primary" or "chief" — like the principal of a school — while principle generally refers to a rule, law, or general truth. Principle is always used as a noun; principal is usually used as an adjective but can also be a noun. "Principal" and "principle" are homophones.
Principal (adjective or noun) conveys the meaning of "primary" or "chief" - like [as adjective] e.g. the principal aim of a series of tasks,or [as noun] the principal of a school. It is also used as the chief part of a loan (i.e., not the interest), as in apply the extra $50 in this month's mortgage payment toward the principal.
Principle (noun) is a rule, law or general truth. The principles of mathematics
These words are sources of confusion for even well-versed minds. Some phrases that can help you remember the difference are:
- The principal alphabetic principle places A before E.
- The school princiPAL is my PAL.
Some writers focus on the distinction that principles can be used in the plural, but principal is usually used only in the singular form. This strategy, when used as a quick rule, often leads to errors. The plural "principals" is appropriate when writing about several individuals, who each have a primary role, whose position of primacy does not conflict with another's similar position. For example,
- The theater company honored the principals." (i.e., their lead actors)
- The school principals gathered for a meeting, but the superintendent did not come.
This video clearly explains the difference between principle and principal:
- first or highest in rank, importance, value, etc.; chief; foremost.
- a chief or head of an organization.
- something of primary importance.
- In Law: a person who authorizes another (an agent) to represent him or her.
- In Finance: a capital sum, the main amount of a loan or investment (as distinguished from interest or profit).
- a fundamental assumption e.g. The principles of physics dictate that you cannot travel faster than the speed of light.
- a generally acceptable rule of action or behavior. Moral principles e.g. I am an environmentalist. I prefer cloth diapers over disposable on principle.