The misuse of the word "theory"

In my verbiage "opening theory" translates into tje principles that should govern ones play in the opening phase of a game. One has to understand and correctly apply these principles. More or less mindlessly memorizing different lines is another matter.

'Develop your pieces' is hardly theory, or could we find out it might be wrong? Chess theory is what is believed to be true after like scientific explorations. Modestly and proudly even lines that have been tested long time are called 'theory'. I mean, one can mindlessly fight for the center and bring one's king to safety and one (not me) can be able to explain the subtle differences in memorized lines.

Yeah, but if one follows a memorized line without knowing the reasons for each move one doesn´t relly apply theory, regardless how sound the line might be. And "develop your pieces" is just one of many governing principles. In practice those principles may conflict, ie a move can be in accordance with one or more principles but conflicting with others. The player who applies hus theoretical knowledge can (hopefully) in such situations (which are the norn rather than exceptions) use his knowledge and judgement to solve this conflict and find the right move. The player who relies on memorized lines is lost once he enters uncharted territory or suffers a lapse of memory.

Ok, theory can be misused, but you said the word 'theory' was misused.. Imagine we give an account of a game "I was so busy about developing my pieces that I totally forgot castling." So we did not correctly apply opening principles. Even if we agree that we could call opening principles 'theory' we can reach unapplicability there as well. We lose track of our checklist and might even be unable to recall it if we try.

Principles seem to be easier to know, opening lines more difficult. To recall only the main line helps when two players can and want to produce it. It is applying a result of theory, even if you don't know why what you play is played. One strange move and you may be in a strange land. But as I said if you brought out all your pieces without knowing what for, you are lost once they are all out, too.

It's idiomatic for "moves considered to be good moves in the opening known from the start of the game", whether or not you understand why. At the board you may come up with the most brilliant move in chess history, and this may become part of opening theory, but it is not opening theory at the time you played it, nor are the reasons you played it "opening theory". Also if you studied an engine before the game, those moves can be considered "theory," whether or not anyone else has ever looked into the move.
When I first encountered this term I thought it was strange. "He's still in theory" means he's still playing memorized moves. Also if the moves made are widely understood to be the best move, whether the player knew it or not, the move can be said to be "theory." So I complete newb playing e5 to e4 is playing theory, even if they have no clue.
This is my understanding.

Arguing about dictionary definitions is a waste of time at this point. Like it or not the term "theory," as in "opening theory" refers to the compendium of what has been played in the past. Ordinarily, we might call that "opening practice," but it's too late now!

It's probably not an ideal word choice, but certainly the true theoreticians are not just memorizing, but know the reasons behind each move. And when there are more than one acceptable move, they know the competing reasons, along the lines of "do this if you want a more basic game (or drawish game or whatever)," and do this other thing if you want a more complicated game (or sharp game or whatever).