A geis or geas (pl. geasa) is an idiosyncratic taboo, whether of obligation or prohibition, similar to being under a vow or curse, yet the observance of which can also bring power and blessings. It is also used to mean specifically a spell prohibiting some action. Geasa are common in Irish and Scottish folklore and mythology, as well as in modern English-language fantasy fiction.

The word originates in Old Irish, also known as Old Gaelic, and retains the same form in Modern Irish (nominative singular geis /ɟɛʃ/, nom. plural geasa /ˈɟasˠə/; genitive sg. geise /ˈɟɛʃə/, gen. pl. geas /ɟasˠ/). In modern Scottish Gaelic, the spelling has evolved in a slightly different direction (nom. sg. geas /kʲes/, nom. pl. geasan, gen. sg. geis or geasa). It has also been borrowed into English in both forms (sg. geas or geis /ɡɛʃ/ or /ˈɡi.əʃ/, pl. geasa)).

A geas can be compared with a curse or, paradoxically, a gift. If someone under a geas violates the associated taboo, the infractor will suffer dishonor or even death. Conversely, the observing of one's geas is believed to bring power. Often, women place geasa upon men; in some cases, the woman turns out to be a goddess or other sovereignty figure.

The geas is often a key device in hero tales, such as that of Cú Chulainn in Irish mythology. Traditionally, the doom of heroes comes about due to their violation of their geas either by accident or by having multiple geasa, which then come into conflict. For instance, Cú Chulainn has a geas to never eat dog meat, and he is also bound by a geas 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.

In some cases, the placing of a geas can lead to tragedy even when it is not violated. Aoife imposed three geasa on Connla, her son with Cú Chulainn: he cannot turn back once he starts his journey; he must not refuse a challenge; and he must never tell anyone his name. She then sent Connla, aged seven, to seek out his father, but he was a child of such extraordinary skill that he was seen as a threat after having defeated all Ulster heroes who met him. Because of the geas placed on him by his mother, he refuses to identify himself, which leads to his own father, Cú Chulainn, killing him in single combat using the Gáe Bulg before recognising too late who he is. He then introduces his dying son to the men of Ulster as a fitting hero.

A geas might appear beneficial by involving a prophecy that a person would die in a particular way so bizarre that they could then avoid their fate for many years.^{[citation needed]} As with Conaire Mór, though, in the tale of Togail Bruidne Dá Derga, who strictly observed a number of geasa, a small unconnected infraction can escalate to one's undoing. By initially making exceptions to crimes of stealing by his foster-brothers contravening fír flathemon, the king's upholding of true judgement, things proceed until they deliberately contravene a geis of Conaire's against marauding in his reign. Though he tries to rectify the situation by exiling them, his fate intervenes, so the remaining geasa are involuntarily and accidentally broken one after the other with a sense of gathering doom that cannot be checked.

In the Irish saga of Conchobar mac Nessa, the king is said to have the right to the first night with any marriageable woman and the right to sleep with the wife of anyone who hosted him. This is called the Geis of the king. Whether this right actually existed and was exercised by the Celts is not attested outside the sagas. It is similar to the droit du seigneur of feudal Europe.

Considerable similarity exists between the Goidelic geasa and the Brythonic tynged. This is not surprising given the close origins of many of the variants of Celtic mythology.