A geasa is a vow or obligation placed upon a person (usually a hero, such as Cuchulainn) in Celtic mythology. Traditionally, the doom of the hero comes about due to his violation of such geasa, usually by accident.
There is a considerable similarity between geasa (which are a phenomenon of Irish Celtic mythology) and the foretold deaths of heroes in Welsh mythology. This is not surprising given the close origins of all the variants of Celtic mythology.
Geasa is also the name of a celtic metal band, see Geasa (band).