Principle  n. 1 fundamental truth or law as the basis of reasoning or action. 2 a personal code of conduct (person of high principle). B (in pl.) Personal rules of conduct (has no principles). 3 general law in physics etc. 4 law of nature forming the basis for the construction or working of a machine etc. 5 fundamental source; primary element.  in principle in theory. On principle on the basis of a moral attitude. [latin principium source]