A rule or standard, especially of good behavior: a man of principle. A basic truth, law, or assumption: the principles of democracy. The collectivity of moral or ethical standards or judgments: a decision based on principle rather than expediency. A fixed or predetermined policy or mode of action. A basic or essential quality or element determining intrinsic nature or characteristic behavior: the principle of self-preservation. A rule or law concerning the functioning of natural phenomena or mechanical processes: the principle of jet propulsion. Chemistry One of the elements that compose a substance, especially one that gives some special quality or effect. A basic source. See Usage Note at principal. in principle With regard to the basics: an idea that is acceptable in principle. on principle According to or because of principle.