Seidhr ("sayth" or "seethe") is a traditional form of visionary soul-magic native to Northern and Central Europe.
Seidhr combines deep empathy, visions and trance with healing, spirit-travel and messages, and energy work. Mastered by the Goddess Freyja, taught to Odin, and found throughout ancient Germanic and Norse lore in the form of history, saga and myth, seidhr is still practiced today by modern Heathens. Historically practiced by holy women known as volva or seidhkona, seidhr was traditionally powerful women's magic-- but can also be performed by men. It is closely related to Siberian practices (from which we get the word shaman), but focuses primarily on Gods, ancestors and webs of energy and fate. If you have ever changed something in a lucid dream (or been someone else in a vivid dream), you have unknowingly practiced seidhr.
The more familiarly known Celtic Druids and some Icelanders had similar practices called utiseta (sitting out, covered up in a sacred or secluded place, often a burial mound, seeking a vision and communication with spirits), and use of the imbas forasnai-- "the light of foresight" to see the future in visions or trance.
Seidhr can be used to meet and draw closer to the Gods and beloved ancestors, build friendly bonds with nature spirits, develop spiritually, heal and clear energies, and to inquire about and alter the web of fate.
Most of the old Norse stories found in the Poetic Edda relate to seidhr journeys. One of the many direct references to seidhr in Norse myth comes from the ancient Icelandic poem Skirnismal, in which the God Frey sits in the kingly "high seat", glimpses the radiant Goddess Gerd in the Underworld, a place he cannot leave to visit himself, and becomes brooding and lovesick until they can be united. Another, from the Prose Edda, involves Loki borrowing Freyja's magical falcon cloak in order to travel to the realm of the Giants in the form of a hawk.
Both the summoning of a volva by Odin and Freyja and the trances of the seers mentioned in Voluspa, Hyndluluid and Baldrs draumar also fall under seidhr practice, as does a historical account from Greenland in The Saga of Eirik the Red.