From Middle English principle, from Old French principe, from Latin prīncipium, from prīnceps ("first").

• IPA: /ˈpɹɪnsɪpəl/, /ˈpɹɪnsəpəl/

principle (plural principles)

1. A fundamental assumption or guiding belief.

   Synonyms: premise

   We need some sort of principles to reason from.

2. A rule used to choose among solutions to a problem.

   The principle of least privilege holds that a process should only receive the permissions it needs.

3. (sometimes, pluralized) Moral rule or aspect.

   Synonyms: tenet

   I don't doubt your principles.

   You are clearly a person of principle.

   It's the principle of the thing; I won't do business with someone I can't trust.

   • 1837, L[etitia] E[lizabeth] L[andon], “The Author and the Actress”, in Ethel Churchill: Or, The Two Brides. […], volume III, London: Henry Colburn, […], →OCLC ↗, page 105 ↗:

     Lavinia—shrewd, careless, clever; ready to meet any difficulty, however humiliating, that might occur; utterly without principle; confident in that good fortune, which she scrupled at no means of attaining—was the very type of the real.

4. (physics) A rule or law of nature, or the basic idea on how the laws of nature are applied.

   Bernoulli's principle

   The Pauli Exclusion Principle prevents two fermions from occupying the same state.

   The principle of the internal combustion engine

5. A fundamental essence, particularly one producing a given quality.

   Many believe that life is the result of some vital principle.

   1. A chemical compound within plant or animal tissue that is characteristic of it and more or less peculiar to it, such that it defines the character of that tissue from a human viewpoint (as for example nicotine in tobacco).

      the active principle

      • 1845, William Gregory, Outlines of Chemistry:

        Cathartine is the bitter, purgative principle of senna.

6. A source, or origin; that from which anything proceeds; fundamental substance or energy; primordial substance; ultimate element, or cause.
   • 1664, John Tillotson, “Sermon I. The Wisdom of Being Religious. Job XXVIII. 28.”, in The Works of the Most Reverend Dr. John Tillotson, Late Lord Archbishop of Canterbury: […], 8th edition, London: […] T. Goodwin, B[enjamin] Tooke, and J. Pemberton, […]; J. Round […], and J[acob] Tonson] […], published 1720, →OCLC ↗:

     The soul of man is an active principle.

7. An original faculty or endowment.
   • 1828, Dugald Stewart, The Philosophy of the Active and Moral Powers of Man:

     those active principles whose direct and ultimate object is the communication either of enjoyment or suffering

8. Misspelling of principal
9. (obsolete) A beginning.
   • 1596, Edmund Spenser, “Book V, Canto IX”, in The Faerie Queene. […], London: […] [John Wolfe] for William Ponsonbie, →OCLC ↗, stanza 2:

     Doubting sad end of principle unsound.

• prince
• principal
• principality

• French: principe
• German: Grundsatz, Prinzip
• Italian: principio
• Portuguese: princípio
• Russian: при́нцип
• Spanish: principio

• French: principe
• German: Prinzip
• Italian: regola
• Portuguese: princípio
• Russian: при́нцип
• Spanish: principio

• French: principe
• German: Prinzip
• Italian: principio, valore
• Portuguese: princípio
• Russian: при́нцип
• Spanish: principio

• French: principe
• German: Naturgesetz
• Italian: principio
• Portuguese: princípio
• Spanish: principio

principle (principles, present participle principling; simple past and past participle principled)

1. (transitive) To equip with principles; to establish, or fix, in certain principles; to impress with any tenet or rule of conduct.
   • 1689 (indicated as 1690), [John Locke], chapter 4, in An Essay Concerning Humane Understanding. […], London: […] Eliz[abeth] Holt, for Thomas Basset, […], →OCLC ↗, book I, page 20 ↗:

     Let an enthusiast be principled that he or his teacher is inspired.