Blunder
A blunder is a very serious error, such as leaving a piece undefended or missing a decisive tactic, that swings the evaluation so far that a good position turns bad or a bad one becomes lost.
Also called: blunder mark
A blunder is the heaviest error grade in game review. Software flags it when your move causes a large drop in the evaluation, such as giving up a piece for nothing, walking into a forced mate or allowing a combination you did not see. At club level, most games are decided by one or two blunders rather than by deep strategy, and both sides usually make some. The causes are often ordinary: hurrying, assuming the opponent's last move was harmless, forgetting that a piece was pinned or that a back rank was weak. Tools differ in how large a swing they require before applying the label. The common mistake after a blunder is to blame bad luck or time. A better habit is to classify it. Was it a calculation slip, a missed threat, or a lapse in concentration? Then build a routine that targets that cause, such as checking all opponent captures and checks before every move. Over time the count of blunders is one of the most useful numbers to watch.
A neighboring idea is Average centipawn loss. Average centipawn loss, or ACPL, is the mean amount of evaluation a player gave up per move compared with the engine's best choice, measured in hundredths of a pawn, so lower figures mean cleaner play.
It sits close to Missed win. A missed win is a game review label for a move that fails to take a clearly winning continuation, so a decisive advantage shrinks to something much smaller or disappears entirely.
A neighboring idea is Blunder check. A blunder check is a quick test, made by a person before moving or by software afterward, that looks for a move which hangs material, allows a tactic or loses the game outright.
Computer terms such as "Blunder" are most useful next to a coach who reads your games with you, which is how a first lesson with George Li works.
Related terms
A mistake is a game review label for a move that clearly worsens a position, costing a meaningful share of the advantage, though it is less severe than a blunder that throws the game away.
An inaccuracy is the mildest error label in game review, given to a move that is playable but gives up a small amount of advantage compared with the engine's better alternative.
A blunder check is a quick test, made by a person before moving or by software afterward, that looks for a move which hangs material, allows a tactic or loses the game outright.
A hanging piece is a piece that is attacked by the opponent and has no defender, so it can be captured for free unless its owner moves it, defends it or makes a bigger threat.
Tilt is a state of frustration or emotional disturbance after a setback, where a player stops thinking clearly, plays faster or more recklessly, and makes worse decisions than usual.
Concentration is the ability to keep your attention on the current position, the opponent's ideas and your own calculation for the whole game without drifting into distraction, hope or worry.
Guides that go further
What to decide first, the questions to ask, what a first lesson should show and the red flags to avoid.
How much does chess coaching cost?
Published rates for private, group and subscription chess coaching, with the source and the date of each.
A game review, a read on your level and a written study plan from George Li.
Want to spot this in your own games?
A first lesson starts with your games: George reviews them and builds a study plan around what he finds.