What is a Blunder?

So...this seems obvious at first.  It's a terrible move.

But I recently had a game that was "full of blunders" by the judgement of the game review function.  And I found it annoying because, well, most of these "blunders" involved tactics playing out at nine or more (sometimes quite a bit more) ply of depth, and not even forcing lines, but branching lines, so I really didn't see any chance that I would ever see these things over the board.

Now, admittedly, some of them I'd like to think I could see, but that's not quite my point right now.

So I started thinking about this word, and I asked myself this:  was Karpov's move 25, in the 1990 World Championship, Game 20, a blunder?  Sure, it's a losing move, giving Kasparov probably his most famous win, after sacrifice, of all time.

But does a blunder mean any losing move, or is it something you should have been able to see?  And what Karpov should be able to see, is clearly not what I should be able to see.

But when I think of it, it means "a bad move I should have seen," not "the tablebase says it's mate in 41."  So, something within five ply generally...maybe a bit more if the line has no branches and is completely forced.

I guess it's just a word...

So even if, it just happens to turn out that a certain move, after a chain of 14 more moves (ply, not moves) with three branches at the key point, will lead to a double attack on two pieces (or in another line, a piece will run out of escape squares)--that's a blunder, same as hanging the piece to be taken in one move?

OK.  If that's what it means.  I just never had the sense it really meant that.

The game review isn't always accurate. It even changes the review sometimes when you visit the game analysis again at a later time. Perhaps you can take a look at the review again and see if anything you recall has been altered.

Yeah, I guess.  I mean, the review is correct as far as it goes.  You have a knight and a queen being successively pushed back and no matter what you do (well, ok, i'm not going to argue with stockfish), you wind up having to lose material.  In one line the queen is denied every square on the third rank by some piece or pawn...it's not that it's wrong...just I never expect to be able to forsee it or do anything about it because it seems like both pieces have many options, just the options successively close down over time.  I dunno.  It just makes me sad because...if that's a blunder there's nothing I can do about it.

Don't let it bother you too much. Just know that when an engine sternly criticizes you for not calculating as good as it does, it's just like a 10th grader making fun of a 3rd grader for not knowing things like quadratic functions and logarithmic equations. Like, how do you even expect them to have the slightest idea of what that is? Always take the game review with a grain of salt because not everything it suggests will apply to you.

First, a blunder is an opinion, a word people put on poor moves that they believe are game losing bad.  Usually people get this right, but sometimes there is an overlooked response esp if only one person is chanting the 'blunder' mantra. 
here is what CDC thinks it is: https://www.chess.com/terms/chess-blunder

However the CDC computer labels lots of things as blunders that are a bit harsh for lower ranked players (eg this loses the game 53 moves later stuff.  Ok, sure, its a blunder, but the guy was rated 325 ... )