The Aesir (pronounced “ICE-ear”; Old Norse Æsir for multiple gods, Ásynjur for multiple goddesses, Áss for one god, and Ásynja for one goddess) were one of the two main tribes of deities venerated by the pre-Christian Norse. (When referring to a group of deities that included both male and female divinities, the masculine plural “Aesir” was used.) The second tribe was the Vanir, who are only nominally distinct from the Aesir; in Old Norse literature, the word “Aesir” is commonly used to refer to the gods as such, without regard for the nominal tribal distinction. Most of the best-known Norse gods and goddesses belong to the Aesir, including Odin, Thor, Frigg, Tyr, Loki, Baldur, Heimdall, Idun, and Bragi. Their home is Asgard, one of the Nine Worlds, which is located in the highest, sunniest branches of the world-tree Yggdrasil. In the Norse sources, Odin, the “Allfather,” is their chief.
The word “Aesir” is almost certainly derived from one of two Proto-Germanic words: *ansaz, “pole, beam, rafter,” or *ansuz, “life, vitality.” In either case, when this linguistic evidence is combined with the Aesir’s role in the tales of Norse mythology, it becomes clear that the Aesir were thought of as being the powers that hold the cosmos together, and prevent the giants from succeeding in their attempt to drag it back into the formless chaos from which it originally came. At Ragnarok, however, the gods are fated to fail, and the giants to succeed.
For more on how the Vikings conceptualized the gods in general, see Norse Theology.
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 Orel, Vladimir. 2003. A Handbook of Germanic Etymology. p. 20-21.