I come across these terms in some papers. My understanding is that it is an equation or model describing a phenomenon. Usually, the equations are given and claimed to be true with only some explanations and justification, but not derived from the first principle. The reason is often a bit obscure to follow, on what base they can do that with high confidence? How can they derive it in the first place? If the only purpose is to generate the expected phenomenon, how can they justify it is really the equation governing the phenomenon?
Is there good definition and explanations of these two terms? I expect few solid physics examples should make it easy to understand.