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. in principle as regards fundamentals but not necessarily in detail. on principle on the basis of a moral attitude (I refuse on principle).
[ ME f. OF principe f. L principium source, (in pl.) foundations (as PRINCE) ]