principle

UK: /ˈprɪnsɪp^əl/ US: /ˈprɪnsəpəl/ ,(prin′sə pəl)

WordReference Random House Learner's Dictionary of American English © 2025

prin•ci•ple /ˈprɪnsəpəl/   n. 

1. a fundamental law that describes how a thing moves, works, or acts:
   [countable] the principles of modern physics.
2. a personal basic rule by which one lives:
   [countable] to stick to your principles and be honest and forthright.
   [uncountable] a man of principle.
3. the method of operating reflected or used in a certain instance:
   [countable] a family organized on the patriarchal principle.

Idioms

4. Idioms in principle, basically;
   fundamentally:
   He favors the plan in principle.
5. on principle, according to rules for right conduct:
   I refused to support her candidacy on principle.

See -prim-.

WordReference Random House Unabridged Dictionary of American English © 2025

prin•ci•ple  (prin′sə pəl), n. 

1. an accepted or professed rule of action or conduct:
   a person of good moral principles.
2. a fundamental, primary, or general law or truth from which others are derived:
   the principles of modern physics.
3. a fundamental doctrine or tenet;
   a distinctive ruling opinion:
   the principles of the Stoics.
4. principles, a personal or specific basis of conduct or management:
   to adhere to one's principles; a kindergarten run on modern principles.
5. guiding sense of the requirements and obligations of right conduct:
   a person of principle.
6. an adopted rule or method for application in action:
   a working principle for general use.
7. a rule or law exemplified in natural phenomena, the construction or operation of a machine, the working of a system, or the like:
   the principle of capillary attraction.
8. the method of formation, operation, or procedure exhibited in a given case:
   a community organized on the patriarchal principle.
9. a determining characteristic of something;
   essential quality.
10. an originating or actuating agency or force:
    growth is the principle of life.
11. an actuating agency in the mind or character, as an instinct, faculty, or natural tendency:
    the principles of human behavior.
12. [Chem.] a constituent of a substance, esp. one giving to it some distinctive quality or effect.
13. [Obs.] beginning or commencement.
14. in principle, in essence or substance;
    fundamentally:
    to accept a plan in principle.
15. on principle:
    • according to personal rules for right conduct;
      as a matter of moral principle:
      He refused on principle to agree to the terms of the treaty.
    • according to a fixed rule, method, or practice:
      He drank hot milk every night on principle.

• Middle English, alteration of Middle French principe or Latin prīncipium, on the analogy of manciple. See principium 1350–1400

Synonyms

• 1, 2.
• 3. Principle, canon, rule imply something established as a standard or test, for measuring, regulating, or guiding conduct or practice. A principle is a general and fundamental truth that may be used in deciding conduct or choice:
  to adhere to principle.
  Canon, originally referring to an edict of the Church (a meaning that it still retains), is used of any principle, law, or critical standard that is officially approved, particularly in aesthetics and scholarship:
  canons of literary criticism.
  A rule, usually something adopted or enacted, is often the specific application of a principle:
  the golden rule.
• 2. theorem, axiom, postulate, proposition.
• 5. integrity, probity, rectitude, honor.

Usage

See principal. 

Collins Concise English Dictionary © HarperCollins Publishers::

principle / ˈprɪnsɪp^əl/ noun

1. a standard or rule of personal conduct: a person of principle
2. (often plural) a set of such moral rules: he has no principles
3. a fundamental or general truth or law
4. the essence of something
5. a source or fundamental cause; origin
6. a rule or law concerning a natural phenomenon or the behaviour of a system: the principle of the conservation of mass
7. a constituent of a substance that gives the substance its characteristics and behaviour
8. in principle ⇒ in theory or essence
9. on principle ⇒ because of or in demonstration of a principle

Etymology: 14th Century: from Latin principium beginning, basic tenetPrinciple and principal are often confused: the principal (not principle) reason for his departure; the plan was approved in principle (not in principal)