Principle
Part of speech: noun
Definition: A fundamental assumption. A rule used to choose among solutions to a problem. Moral rule or aspect. A rule or law of nature, or the basic idea on how the laws of nature are applied. A fundamental essence, particularly one producing a given quality. A beginning.
Theory
Part of speech: noun
Definition: An unproven conjecture.An expectation of what should happen, barring unforeseen circumstances.A coherent statement or set of statements that attempts to explain observed phenomena.A logical structure that enables one to deduce the possible results of every experiment that falls within its purview.A field of study attempting to exhaustively describe a particular class of constructs.A set of axioms together with all statements derivable from them.