n.1 a fundamental truth or law as the basis of reasoning or action (arguing from first principles; moral principles).
2 a a personal code of conduct (a person of high principle). b (in pl.) such rules of conduct (has no principles).
3 a general law in physics etc. (the uncertainty principle).
4 a law of nature forming the basis for the construction or working of a machine etc.
5 a fundamental source; a primary element (held water to be the first principle of all things).
6 Chem. a constituent of a substance, esp. one giving rise to some quality, etc.
Phrases and idioms:
in principle as regards fundamentals but not necessarily in detail. on principle on the basis of a moral attitude (I refuse on principle).
Etymology: ME f. OF principe f. L principium source, (in pl.) foundations (as PRINCE)