Joined: Mar. 2005
To put in my two cents here, I would think that either law or principle would be a concise statement about something that happens under the described circumstances. A Theory is a body of knowledge for which the main principle(s) have been confirmed by testing, i.e., a confirmed hypothesis. A hypothesis is a proposal for a new law or principle that hasn't yet been confirmed.
I'm unsure if there's a technical distinction between "law" and "principle"? Would "law" be more general while "principle" could also refer to something that applies in more limited circumstances?
Unfortunately, several of the terms do get misused. String "theory" as I understand it is still unconfirmed, so it should be called "hypothesis", but it's been called "theory" for so long now that this misuse seems very unlikely to get corrected. Not to mention that those unfamiliar with scientific usage think "theory" means hypothesis or even just a guess.
Unfortunately, given all that, if talking to people who wouldn't be sure of the status of the theory under discussion, I'd say to clarify the term, as in "theory confirmed by repeated testing" or "hypothesis that hasn't yet been confirmed", or something along one of those lines.