Define theory

Could someone give a definitive definition of theory as it applies to openings?

I hear it used as people describe openings. Things like:

"We're still in theory"

"This move is part of a system. There's no theory"

"Stay away from the Sicilian. There's loads of theory"

I'm assuming it means a particular move is theoretically sound as opposed to being arbitrary or speculative, but I'm probably wrong.

What's a concrete and easy to understand definition, because it's bugging me that I don't quite get it.

Opening theory refers to a set of moves that are well known and publicised. For example. The French Defence Advance Variation arises from the move order. 1.e4 e6 2.d4 d5 3.e5  The move e5 advances the pawn on e4 thus it is named the 'advance variation'.

With your question on a system, this refers to a series of moves that arrive at the same position regardless of what the opponent plays (under reasonable circumstances) and can arise from varying move orders.

"Stay away from the Sicilian. There's loads of theory" means that professional chess players and theoreticians has concluded that a certain amount of moves are the best or a playable in the position. Of course theory can be prepared by the individual for example some one may find a new move in the theoretical position and analyse it thoroughly. The Sicilian Defence is very popular at top level and as such, many people have contributed to it's opening theory, making it highly theoretical.

Thanks for that.

You explained system very well. Makes perfect sense.

So in essence theory just means moves that have been scrutinized by the world's best players and chess writers to be superior to other moves that could have been made in that particular position?

Moves that have proven to lead to some sort of concrete advantage or at the very least don't give up an advantage to your opponent?

Is that a fair summation?

Yes that's the basic jist of it. But the best theoretical moves don't always lead to a distinct advantage. And theoretical moves don't always have to be the best, or else there would be only a few named variations in contrast to the hundreds of variations in reality.

Since you can sidestep theory with a single non-theoretical move, it is most beneficial to understand the theoretical moves being played in order to understand the positional plans and nuances associated with it. But quite often people don't want to memorize tons of opening theory which is why they would play a system. E.g.

"Theory" is the sum total of all the published opening analysis out there. If it's in a book, or someone has analysed it on a website, or somebody has annotated some GM game, etc, then it's theory.

For instance, the Sicilian has a lot of theory because there are entire books that only start at some position after move 10, and then use 300+ pages to analyse it...

And I think the statement "This move is part of a system, there is no theory" is just wrong. There is plenty of theory on "system" openings.

I believe chessbase reports use the term in that sense, but I think there should be a glimpse of "being reasonably sound" in it too; no one calls a blunder theory.

the statement "theory says ... here", should mean it's a trustworthy move or sequence.

I agree. There are openings that are "theoretically refuted" 

Theory = pre-analyzed.

Obvious blunders hardly need 'pre-analysis' they refute themselves over the board but some are so well known they are theory all the same, scholar's mate and fool's mate are part of theory... known looks-good-but-works-out-bad-moves (i.e. traps) certainly qualify as theory. Theory is known moves, good and bad. Good moves, of course, tend to merit far more investigation and interest than bad moves.  

O.K.  Interesting but, being a novice here, how do you tell good moves from bad moves. i.e. I make a lot of bad moves and know it too late?                                              I have a book of chess openings that is close to 1000 pages and it is a brain burner but it does isolate down to example games that tend to help a smidgen if you pay attention and really, really look at it.