What is "luck" in chess?

Yeah, like this one:

http://www.chess.com/livechess/game?id=1233489682

I thought for sure I would lose this game down all of this material and such and I magically won

While luck is a murky topic an example might be Euwe beating Alekhine or Kramnik beating Kasparov.  However, although Kasparov was the stronger player overall Kramnik knew most people at the time grossly underestimated the Berlin Wall and took advantage of that, so lots of skill obviously came into play in the result there too.  

Fischer, although he was clearly superior to Larsen, winning 6-0 against him could be considered lucky since Fischer wasn't that much better than him.  Noticeably yes, but not that drastically.  

Those aren't really examples of luck. Both Euwe and Kramnik were underdogs, but they defied the odds by being extremely well prepared. It wasn't "luck" that led Kramnik to play the Berlin. Also, Kramnik got into the best shape of his life. If anything, Kasparov was lucky he didn't lose by a larger margin! Kramnik dominated the match.

Euwe and Kramnik were both lucky to be given their title matches. Euwe lost a match to Capa and wasn't seen as the #1 challenger but was still given the title match partly because Capa was better. Kramnik lost the Candidates match to Shirov but Kasparov gave Kramnik the title match anyway.

It wasn't luck that made Kramnik play the Berlin obviously, only luck (not 100% luck but some) that the decision paid off.  

I just played someone on a 10 minute blitz game. Both rated around 1400. My opponent made a hilarious error when the game was level, all i had to do was take the rook with my queen for checkmate. But i didn't see it. Well, i did. About 1 second after i'd already moved! Is that luck?

I still won the game so...

Yeah, I mean when you broaden your definition, what isn't luck? Everyone has ups and downs regardless of what activity they are pursuing, but that doesn't mean that what makes them do what they do is random or unconnected to them...

It was indeed very impressive match play by Kramnik, wasn't it?

http://www.chess.com/chessmentor/view_lesson?id=684

While the winner of an indivdual game may not attribute his win to luck, the winner (and losers) of tournaments often explain that they got lucky, or unlucky. 

Here's what Alekhine said after the AVRO tournament:

 "When we have said this, and have added that the tournament revealed no player who outclassed the rest (the first two prizes were tied for), there still remains a salient fact of which the chess world will have to take account—the victory of youth. We may try to explain or excuse the ill-success of one player or another by special circumstances, such as fatigue or ill-health; we may throw doubt on the superiority of a tournament winner, since success is almost always partly a matter of luck; but in all fairness, we cannot get away from the fact that three representatives of the younger generation beat the world champion and his two predecessors."

http://www.theweekinchess.com/john-watson-reviews/john-watson-book-review-105-biographies-and-game-collections-2

Luck in chess is the resources hidden beyond the players' (human or silicon) calculation horizon. The further you calculate the more you can minimize the impact of luck, but it will always be there.

Sure, I can understand luck in a tournament, but luck as a whole in chess, not really. Even, say, tennis is some luck because some draws are better than others, maybe a ball doesn't go where you thought it would, whatever. But in general, how seriously are we going to take that kind of thinking?

Luck is everything that benefits a player that was not down to their specific skill. This is not to diminish the skills that players have or to attempt to claim that opponent's just got lucky when they won.

If an player A happens to play an opening that player B knows very well then player B is benefiting from luck... if player B had looked through every single game of player A's that they could find to see which openings they prefer then player B is benefiting from the specific skill of diligence... though they may still be lucky that player A was less diligent.

Absolute skill would be to be able to play as well as a (hypothetical) 32 man tablebase.

In chess, the more skilled player will probably win, but not always (if the skill levels aren't too different). What else could make it 'probably' rather than 'definitely' if not chance/luck?

for example when your opponent plays one of the very few opening lines you dont know good enough and just beats you with this while you would have won with any other opening. its unlucky.

A simple demonstration of luck in chess is a match between engines. It takes many games to determine which engine is stronger between, say, Stockfish and Houdini. Most games between the two will end In a draw, but Stockfish will eventually win more than it loses. We can assign a probability score for any single game, but all three results are still possible.

While chess isn't as random as a roll of dice or a card game, luck still plays a role. The stronger the player, the smaller that role. But it can never be eliminated.

Because the amount of skill you demonstrate can vary. If you demonstrate less skill than you normally demonstrate, indeed that will contribute to your loss.

And remember that "skill" can be kind of ambiguous. Rating tends to denote "overall skill," but of course in reality there are many different combinations of skills you can have. You can have more skill in certain types of positions than others for example. You could say that the measure of overall skill doesn't tell you everything because you don't know if the position will suit one player or the other. Sure. But all that means is that "overall skill" is an imprecise measure of skill. If you wanted the real deal, you would look at a person's individual "skill values" for each different type of position (which could be dozens, hundreds). Of course in practice we can't do such a measurement very well.

the total luck in chess is zero, as your good luck is the others bad luck. local luck depends on preparation, as chance favours the more prepared mind.

From chessopedia, we have an account of Robert Huebner whose brush with luck was really remarkable: 

Robert Heubner, born Nov 6, 1948, is a German Grandmaster (1971) and strongest German player since World War II.  In 1967 he won the German championship.  In 1971 he was playing in a candidates match with Petrosian when he made a mistake in his 7th game.  He overlooked a winning move, became demoralized after he saw the mistake, resigned, burst into tears, and withdrew from the match.  He said he was bothered by street noises.  Petrosian merely turned his hearing aid down.  In 1982 he tied in a match with Smyslov in a Candidates match.  To break the tie, both players agreed to use a roulette wheel to select a winner.  Huebner’s color was black and Smyslov’s color was red.  The wheel was spun and it came up green (0).  A second spin came up red (3) in Smyslov’s favor.< Huebner was thus eliminated. >   Huebner is a papyrologist and has a PhD.  He is also a world-class player in Chinese chess (Xiangqi).  He learned how to play chess from his father at the age of 5.