Blunder

A blunder is a spectacularly bad decision with detrimental consequences to the party that makes it. The term is often used to refer to military, diplomatic, political, or business decisions.

Examples of blunders include:

• British policy toward the United States before the American Revolutionary War
• Decision making prior to World War I
• Mussolini's decision to ally with Hitler and subsequently to invade Greece
• The Japanese attack against Pearl Harbor
• U.S. policy during the Vietnam War

In chess, it is a very bad move, often given the '?' or even '??' sign. But what is a blunder, also depends on the player, since a lesser move for a club player may be called a blunder if a Grandmaster plays it.