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In Irish mythology and folklore, a geis (//; [ˈɟɛʃ]; plural geasa) is an idiosyncratic taboo, whether of obligation or prohibition, similar to being under a vow or spell. The Scottish Gaelic spelling "geas" is also common.
A geis can be compared with a curse or, paradoxically, a gift. If someone under a geis violates the associated taboo, the infractor will suffer dishonor or even death. On the other hand, the observing of one's geasa is believed to bring power. Often it is women who place geasa upon men. In some cases the woman turns out to be a goddess or other sovereignty figure.
The geis is often a key device in hero tales, such as that of Cúchulainn in Irish mythology. Traditionally, the doom of heroes comes about due to their violation of their geis, either by accident, or by having multiple geasa and then being placed in a position where they have no option but to violate one geis in order to maintain another. For instance, Cúchulainn has a geis to never eat dog meat, and he is also bound by a geis to eat any food offered to him by a woman. When a hag offers him dog meat, he has no way to emerge from the situation unscathed; this leads to his death.
A beneficial geis might involve a prophecy that a person would die in a particular way; the particulars of their death in the vision might be so bizarre that the person could then avoid their fate for many years.
For example, the Welsh hero Lleu Llaw Gyffes (in one version of his story) was destined to die neither "during the day or night, nor indoors or outdoors, neither riding nor walking, not clothed and not naked, nor by any weapon lawfully made." He was safe until his wife, Blodeuwedd, learning of these foretold conditions, convinced him to show her how he could theoretically be stepping out of a river onto a riverbank sheltered by a roof and put one foot on a goat, and so on, thus enabling the conditions that allowed him to be killed.
Prohibitions and taboos similar to geasa are also found in more recent English literature, though they are not described as geasa in those texts. For example, in William Shakespeare's Macbeth, Macbeth believes himself safe because "no man of woman born shall harm Macbeth." Macduff, an enemy, was "from his mother's womb untimely ripp'd" (i.e., born by Caesarean section) and was therefore not "of woman born".
Another example is in Terry Pratchett's Sourcery where Nijel (sic) the Destroyer (a Barbarian) was under a Geas whereby he could not die until he did something brave. (neither he nor any other character quite knew what a Geas is, but they believed it may be a species of flightless bird)
Charles Stross uses the term extensively in the Bob Howard series to refer to a bond or prohibition.