prin·ci·ple

[prin-suh-puh l]

–noun

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. Chemistry . a constituent of a substance, especially one giving to it some distinctive quality or effect.

13. Obsolete . beginning or commencement.

—Idioms
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.