A blunder is a spectacularly bad decision or action, a mistake or error with detrimental consequences to the party that makes it. It is typically attibutable to faulty perception: the result of not reading signs, or misinterpreting available information. Naturally many sensible decisions, which even in retrospect were carefully taken, may also prove disastrous mistakes.

The term blunder is often used to refer to military, diplomatic, political, social or business decisions. The word comes from the Old Norse blundra "shut one's eyes" in the oldest sense in Middle English, "to stumble around blindly" all from a presumed an Indo-European base *bhlendh- that also gave us "blind." This modern sense is dated from 1711.

A less consequential blunder is a faux pas, or a blooper.

With its background in "blindness" a blunder must be seen in the context of what would have been considered possible at the time.

Science

In science, plain mistakes are in a sense unavoidable. Some blunders may turn out to have positive consequences. Einstein called his introduction of the so-called cosmological constant the "biggest blunder" of his life, and later abandoned the idea. His comment was because he introduced it to maintain a static universe, not long before the observational evidence turned in favour of expansion. Nowadays this constant is needed, to explain the increasing rate of the expanding universe.

Games

In chess, a blunder is a very bad move, often given the '?' or even '??' sign (see chess punctuation). But what counts as 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. In go, there is a slightly more nuanced Japanese language term poka, meaning an unworthy slip by a top-level player. Fujisawa Hideyuki , in terms of pure talent one of the greatest go players, was famous for poka.