What is the definition of a blunder?

Is a blunder only a bad move that leads to direct punishment in the next move or also if the punishment will come after 2-4 moves?

Although many now use it as a synonym for mistake, I always used it to mean any ?? move.  Apparently though that doesn't seem to be the general practice anymore.

I think I read somewhere that the UCI protocol suggests that engines should flag a move as a blunder if the evaluation were to drop by 1 pawn or more.

blunders are not necessarily accompanied by punishment

That seems like a good definition

Ok good but after how many moves? is a blunder also a move which leads to some sort of punishment which may not have been directly perceivable?

no, not all blunders lead to punishment. I see plenty of that. But they should.

any number of moves as longs they are forced probably

There's no limit to how far into the "future" you have to look before a blunder becomes a blunder. An engine would find a blunder knowing the previous position's evaluation, and compare it to the current position's evaluation, and if the difference is past a certain threshold (a little over a pawn), then the last move made was a blunder.

(By "evaluation of a position", I mean the result of the static evaluation of the PV.)

To me a blunder has always meant hanging a piece (at least).

stalemating when 9 Queens up could be a blunder, but i don't think there's a person in this world could explain that to "monster_with_no_underpants"

I've only heard rumors of that guy (never seen his work in person though).

A blunder is a move that makes you want to bang your face on the table or a nearby wall and actually manages to convince you that that is the exact right course of action

worse than ciljettu Andy, unless there is a disability

where I wrote face I intended the word forehead but I guess in practice it does not make much of a difference

I guess what is a blunder also depends on the strength of the player.

The problem I have with the engine definition is that the engine can look deeper into "the future" than most players can. Such a move which is called a blunder by the machine is obviously a bad move, but wouldn't a blunder be something which is definitely more obvious?

Bear in mind though I kinda liked cj... 

That's what I've always said/thought...but who knows in the world of semantics?

I think this is a good question. I always regarded a blunder as a move that drops a piece or loses the game. However the computer here on chess.com regards a move that misses a tactic as a blunder. Not necessarily just a move that loses material.

I think heinzie's definition is most appropriate.

It's simple:  a blunder is a bad move that substantially worsens your position.  If you lose a pawn for no compensation that qualifies as a blunder.  It doesn't matter whether your opponent takes advantage of the blunder.  It doesn't matter if your opponent takes 15 moves before he finally punishes you for the blunder - it's still a blunder.