Meaning and origin of 'theory' in 'opening theory' What precisely does 'opening theory' mean? Generally when I see the word 'theory', I'd expect a conceptual move away from the particular towards the universal. That is, I would expect 'opening theory' to refer to a generalizing reasoning about strategic options and dangers implicit in an opening, aiming for the formulation of guidelines and principles. But the way I see it used in the chess world, 'opening theory' seems to pertain to the memorization of particular variations. Is this correct? If so, I find this slightly odd: How did this use of the word come about?
You are right in your assessment of the usage.

Opening theory refers to particular variations; but also to typical plans specific to the respective openings.

Opening principles (develop pieces, occupy the center...) are of a more general nature applying to all (or most) openings and is closer to what one would consider a "theory".

I don't have a good answer to your question, but in recent years at least there has been a trend away from playing based on opening principles towards  concrete play. So perhaps at one point in time, "opening theory" was a lot more based on principles and less concrete lines!?