In chess, a blunder is a critically bad move or decision that severely worsens the players situation by allowing a loss of material, checkmate, or anything similar). It is usually caused by some tactical oversight, whether due to time trouble, overconfidence, or carelessness). Blunders are most common in beginner games, but all human players make them, even at the world championship level). What qualifies as a "blunder" rather than a normal mistake is somewhat subjective, and weak moves from novice players might be explained by the players lack of skill, while the same move from a master might be called a blunder). Blunders often occur because of a faulty thought process where players do not consider the opponents forcing moves, such as checks, captures, and threats).

One technique formerly recommended to avoid blunders was to write down the planned move on the score sheet, then take one last look before making it. However, in 2005 the International Chess Federation (FIDE) banned it, instead requiring that the move be made before writing it down). To reduce the number of blunders, players can analyze their games, identify their bad moves, and learn from their own mistakes.

In summary, a blunder in chess is a move that negatively affects a players position in a significant way, often leading to a loss of material or checkmate. It is usually caused by some tactical oversight and can occur at any level of play.