Definitions of principle:

• noun:   a basic truth or law or assumption

  Example: "The principles of democracy"

• noun:   a rule or law concerning a natural phenomenon or the function of a complex system

  Example: "The principle of the conservation of mass"

• noun:   a basic generalization that is accepted as true and that can be used as a basis for reasoning or conduct

  Example: "Their principles of composition characterized all their works"

• noun:   a rule or standard especially of good behavior

  Example: "A man of principle"

• noun:   rule of personal conduct
• noun:   (law) an explanation of the fundamental reasons (especially an explanation of the working of some device in terms of laws of nature)

  Example: "The principles of internal-combustion engines"