Principle

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

References:
"principle." The American Heritage® Dictionary of the English Language, Fourth Edition. Houghton Mifflin Company, 2004. Answers.com 07 Mar. 2007. http://www.answers.com/topic/principle