What should the practical meaning of "blunder" be? There have been questions about the reliability of computer evaluation, and famous cases in which computers miss-evaluate positions have been given. What I want to ask is more related to the practical aspect of the evaluation, rather than its effectiveness.

As humans, we may find moves that put our opponents in deep trouble, or lead to positions we may personally enjoy, even though computers may label this moves as blunders. A classical example is Tal's knight sacrifice in the sixth game of their match. Humanly speaking, the move may be considered brilliant due to the creative inspiration it shows, and the practical effects of it (for the position gets complicated and rich in tactical motives, in which Tal felt comfortable). To a computer, though, it is disastrous; artificial intelligence can not feel discomfort nor prefer certain positions above some others, and quickly finds the correct refutation to the sacrifice.

In a practical sense, for a player looking to eliminate blunders (which is a goal dictated to many beginners), what does this actually mean? Is it correct to label as a blunder all refutable moves that severely worsen one's position, as engines do? Or is it more appropriate to use the word blunder in a more pragmatical sense, by referring to refutable moves that severely worsen one's position, and whose refutation can be actually achieved by a human opponent? In other words, how pure should the concept of blunder in chess be? 

I know the question is somewhat philosophical, and I do not expect definite answers.
The definition of the word is rather subjective, since it depends on how much you value objectivity vs practicality. I think it seems reasonable to take both aspects into account. For example, a move that gives up a pawn on the spot (and offers no compensation) could be considered a blunder. Objectively it's clearly a poor move, and practically it's easy to refute: the opponent just takes the pawn, and that's the end of the story.

Meanwhile, a move that objectively loses 2-3 points, but forces the opponent to play 10 difficult moves in a row in order to exploit it (and if they don't play these moves, they will be much worse), may be considered "interesting" or "dubious". Objectively the move is even worse than the previous example, but practically there's a high chance it will work.

Judging the practicality of a move also depends on the time control of the game. In a blitz game, you can get away with a lot more than you could in a long OTB game. But you could also get away with a lot more in an OTB game than you could in a correspondence game with engines. The example above with giving up 2-3 points would definitely be labelled a blunder in correspondence.

So all in all, a blunder could be considered a move that is clearly objectively bad, but also rather easy to refute in an actual game.