. "a request"
Dineen's Irish Dictionary defines it as "A bond, spell, prohibition, a taboo or a magical injunction, the violation of which lead to misfortune and/or death."
However, it is properly defined as a "request" placed upon a warrior by a druid at the time of his or her birth, or at the time of his or her initiation, the breaking of which usually results in death.
This "request" can be anything. Diarmud was under a geas never to refuse to protect a woman; this leads to his trouble with his uncle Fionn, when Fionn's wife Grainne demands Diarmud protect her, and they fall in love. The geas can be dietary--Cuchulainn was forbidden to eat dogmeat (a common dish in ancient Ireland, actually). The geas can seem arbitrary or supernatural--Conair Mor had numerous gease placed on him, from forbidding him to kill birds because his father was the King of Birds, to not being allowed to leave Temhair for more than nine days at a time. His breaking of these gease in "The Destruction of Da Derga's Hostel" lead to his death.
Often, a person didn't know they had a geas placed upon him, and only learned after it had been broken. Other times, two gease which at outset seemed unrelated would end up contradicting each other, leading to the hero inevitably breaking one of them. For instance, the dietary geas of Cuchulainn against eating dogmeat stood in direct contradiction to his geas against refusing a meal. The Morrigan knew this, and disguised herself as an old woman, offering Cuchulainn a stew of dogmeat. Cuchulain had no choice but to eat the dogmeat. This example also shows a certain totemic element also, as Cuchulain's name means "Hound of Cullen." This totem animal aspect is also seen in Conaire Mor's bird prohibition.
And so, a geas is often either religous/totemic, or it is a strict enforcement of a social code (i.e., not refusing to accept or give hospitality). However, it also shows the problems that arise when we are unable to fulfil this requirement. For instance: my mother tells me that she was raised to always be hospitible, to eat whatever the host places before her--a social obligation. However, on Fridays, she was forbidden to eat meat--a religious obligation. What was she to do? Which one do you break? So my mother, risking hell, broke the religious obligation, so as to maintain a social obligation. This may look petty at first, but then she later justified it by saying that she would do more harm by hurting the host and ruining the party, than by fulfilling this religious obligation, which was a more personal obligation--by breaking the personal, religious obligation, she would only hurt herself, as opposed to hurting others by breaking the social obligation.
If she were Cuchulainn, however, God would have her tied to a tree and hacked to death. Such is the nature of the geas.