A particular form of magic especially associated with the god Odin, according to the description offered by the 13th century Icelander Snorri Sturluson:

Odin governed and practiced that art which is most powerful of all, called seidr, and through it he knew the fate of men and future pitfalls, as well as how to bring death on someone, or bring bad luck or cause illness, and how to deprive someone of power and wisdom, and give it to someone else. But such dishonor was associated with this skill that men believed that they could not practice it without dishonor, and so they taught this art to the priestesses.

A skaldic verse from the mid-tenth century by Icelandic poet Kormak Ogmundarson, cited by Snorri, blankly asserts of Odin's seduction of the giantess, Rind, that 'Odin won Rind by seidr'. Elsewhere, however, Snorri claims that it was Freya who was responsible for bringing seidr from the Vanir to the AEsir, and the practice of seidr, with the exception of Odin, seems particularly restricted to women. Of the mysterious female figure of Gullveig, or Heid, who may be Freya in another form, and whose appearance among the AEsir seems to sow such discord, the eddic poem Volsupa says simply:

They called her Heid, when she came to the house
a sibylline witch, who knew the skill of wands,
she practiced seidr where she could, practiced seidr in a trance;
she was always a delight to wicked women

In the Icelandic sagas, particularly the Fornaldarsogur, seidr is clearly the preserve of witches, who can perform transformations and prophecies through its agency, for example the transformation of Signy described in the legendary Volsunga Saga.