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?
In common usage, "theory" often refers to knowledge gained by someone authoritative and at least somewhat verified by a process of critique and reasonably accepted by experts in the field, which you then study/copy/apply as required to solve a problem. It is sometimes used as a synonym for "book-knowledge", which stands opposed to "practice", where you obtain knowledge through invention/experimentation yourself.
This use of the word makes no distinction about the kind of knowledge so obtained. Both specific variations of an opening, as well as underlying principles and techniques, can be defined as "theory".
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!?
I view it as theory as opposed to practice, where practice is what happens in actual games.
Theory is developed at home, published in books, debated in articles, and so on.
It's also a search for absolute truth (as opposite to practical chances); in particular, it's a quest to find a white advantage -- when full equality for black is proven to be available by force from some position, the theoretical interest in it is over. We already know plenty of ways to get an equal game with white.
A move may lead to an advantage according to the latest theoretical articles but not chosen by grandmasters in actual games because the positions, even if slightly better, are just too hard to play correctly. And on the other hand an opening may be completely uninteresting to opening theory because it clearly leads to an equal position, but there may be so much play left in the position that it's very popular in actual play.
Opening books are often produced as works of theory -- they're attempts to improve on the analysis that's been done before, white strives for an advantage, black for equality, and the analysis goes much deeper than is useful for the average reader. Although the trend nowadays is away from that, towards books that explain and instruct more to help the practical player get better results -- even though theory couldn't care less.
I think the word theory, in the context of chess, means what is thought to be the best line. It has a parallel to comparing an experimental result to the theoretical one, the one that's suppose to be correct. Like scientific theories, they are corroborated by a lot of evidence, but sometimes people find a hole and theories are proven wrong. This happened a lot when computers became strong enough to aid professionals in their theoretical work, and many pre computer analysis was shown to be sub-optimal. Of course, it's not all about what computers think, but also what type of positions you like, which line brings the most winning chances... And computers are much weaker in the opening anyways(depending on the position), but they do help to give inspiration and check tactics especially.