prin·ci·ple 
(prĭnsə-pəl)
(prĭnsə-pəl)n.
Idioms: 1. A basic truth, law, or assumption: the principles of democracy.
2.
a. A rule or standard, especially of good behavior: a man of principle.
b. 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.
in principle
With regard to the basics: an idea that is acceptable in principle.
on principle
According to or because of principle.
[Middle English, alteration of Old French principe, from Latin prīncipium, from prīnceps, prīncip-, leader, emperor; see per1 in the Appendix of Indo-European roots.]