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.

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Meaning Adjective - Main; Primary; First; Most important in a group. Noun (pronominal) - Chief (head person); First or main amount (money). Noun - A rule; a law or a law of nature; a fundamental assumption; a moral rule; the main way of doing something.
Part of speech Usually Adjective. Sometimes stands as a Noun (as in the Principal [of a school]). Noun (only).
Synonyms Chief, main, premier, leading, first. Rule, Axiom, Precept, Doctrine, Moral.
Antonyms Unimportant, Trivial, Secondary, Auxiliary. An exception; oddity; irrelevance.
Usage (Marked/Unmarked) Most used (the normal or unmarked word). Specific (the more specific word, the unusual or marked word).

edit Principal vs. Principle Definitions

Principal, which can be used as an adjective or noun, conveys the meaning of "primary" or "chief." For example, there may be "principal tasks" (adjective use) that an employee must see to before others, and "schools have principals" (noun use). Principal also has an alternative meaning as a noun. It can refer to 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."

Meanwhile, principle is only ever a noun that refers to a rule, law, or general truth (e.g., the rules or principles of mathematics).

This video clearly explains the difference between principle and principal:

edit Usage

Knowing when to use principal vs. principle can be difficult. Some remember the difference with the help of clever, memorable phrases:

Others 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, can often lead to errors. Not only does principle frequently appear in singular form, but the plural "principals" is also appropriate when writing about several individuals, who each have a primary role, whose position of primacy does not conflict with another's similar position (e.g., "The school principals gathered for a meeting, but the superintendent did not attend.").

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"Principal vs Principle." Diffen LLC, n.d. Web. 28 Jul 2015. < >