Pronunciation: (prin'su-pul),[key] —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, a. 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. b. according to a fixed rule, method, or practice: He drank hot milk every night on principle.