A principle is a rule which tells whether one solution is better than another one with respect to a certain aspect.
A principle names and explains a certain aspect to consider while assessing a possible solution to a problem. It can be documented using principle descriptions and help talking about solutions.
Principles are a general and universal way to communicate experience. They describe a certain aspect leaving out all the other aspects. So a principle is not a rule that is valid in every case. It's a certain tendency, a rule of thumb or a heuristic that only in combination with other principles gives a complete picture.