Odin (/ˈoʊdɪn/; from Old Norse: Óðinn) is a widely revered god in Norse mythology and Germanic paganism. Most surviving information on Odin comes from Norse mythology, but he figures prominently in the recorded history of Northern Europe. This includes the Roman Empire's partial occupation of Germania (c. 2 BCE), the Migration Period (4th–6th centuries CE) and the Viking Age (8th–11th centuries CE). Consequently, Odin has hundreds of names and titles. Several of these stem from the reconstructed Proto-Germanic theonym Wōðanaz, meaning "lord of frenzy" or "leader of the possessed", which may relate to the god's strong association with poetry.

Most mythological stories about Odin survive from the 13th-century Prose Edda and an earlier collection of Old Norse poems, the Poetic Edda, along with other Old Norse items like Ynglinga saga. The Prose Edda and other sources depict Odin as the head of the pantheon, sometimes called the Æsir, and bearing a spear and a ring. Wider sources depict Odin as the son of Bestla and Borr; brother to Vili and Vé; and husband to the goddess Frigg, with whom he fathered Baldr. Odin has many other sons, including Thor, whom he sired with the earth-goddess Jörð. He is sometimes accompanied by animal familiars, such as the ravens Huginn and Muninn and the wolves Geri and Freki. The Prose Edda describes Odin and his brothers' creation of the world through slaying the primordial being Ymir, and his giving of life to the first humans. Odin is often referred to as long-bearded, sometimes as an old man, and also as possessing only one eye, having sacrificed the other for wisdom.

Odin is widely regarded as a god of the dead and warfare. In this role, he receives slain warriors—the einherjar—at Valhöll ("Carrion-hall" or "Hall of the Slain") in the realm of Asgard. The Poetic Edda associates him with valkyries, perhaps as their leader. In the mythic future, Odin leads the einherjar at Ragnarök, where he is killed by the monstrous wolf Fenrir. Accounts by early travellers to Northern Europe describe human sacrifices being made to Odin. In Old English texts, Odin is euhemerized as an ancestral figure for royalty and is frequently depicted as a founding figure for various Germanic peoples, such as the Langobards. In some later folklore, he is a leader of the Wild Hunt, a ghostly procession of the dead.

Odin has an attested history spanning over a thousand years. He is an important subject of interest to Germanic scholars. Some scholars consider the god's relations to other figures—as reflected, for example in the etymological similarity of his name to the name of Freyja's husband Óðr. Others discuss his historical lineage, exploring whether he derives from Proto-Indo-European mythology or developed later in Germanic society. In modern times, most forms of the new religious movement Heathenry venerate him; in some, he is the central deity. The god regularly features across all forms of modern media, especially genre fiction, and—alongside others in the Germanic pantheon—has lent his name to a day of the week, Wednesday, in many languages.

The Old Norse theonym Óðinn (runic ᚢᚦᛁᚾ on the Ribe skull fragment) is a cognate of other medieval Germanic names, including Old English Wōden, Old Saxon Wōdan, Old Dutch Wuodan, and Old High German Wuotan (Old Bavarian Wûtan). They all derive from the reconstructed Proto-Germanic masculine theonym *Wōðanaz (or *Wōdunaz). Translated as 'lord of frenzy', or as 'leader of the possessed', *Wōðanaz stems from the Proto-Germanic adjective *wōðaz ('possessed, inspired, delirious, raging') attached to the suffix *-naz ('master of').

Internal and comparative evidence all point to the ideas of a divine possession or inspiration, and an ecstatic divination. In his Gesta Hammaburgensis ecclesiae pontificum (1075–1080 AD), Adam of Bremen explicitly associates Wodan with the Latin term furor, which can be translated as 'rage', 'fury', 'madness', or 'frenzy' (Wodan id est furor : "Odin, that is, furor"). As of 2011, an attestation of Proto-Norse Woðinz, on the Strängnäs stone, has been accepted as probably authentic, but the name may be used as a related adjective instead meaning "with a gift for (divine) possession" (ON: øðinn).

Other Germanic cognates derived from *wōðaz include Gothic woþs ('possessed'), Old Norse óðr ('mad, frantic, furious'), Old English wōd ('insane, frenzied') and Dutch woed ('frantic, wild, crazy'), along with the substantivized forms Old Norse óðr ('mind, wit, sense; song, poetry'), Old English wōþ ('sound, noise; voice, song'), Old High German wuot ('thrill, violent agitation') and Middle Dutch woet ('rage, frenzy'), from the same root as the original adjective. The Proto-Germanic terms *wōðīn ('madness, fury') and *wōðjanan ('to rage') can also be reconstructed. Early epigraphic attestations of the adjective include un-wōdz ('calm one', i.e. 'not-furious'; 200 CE) and wōdu-rīde ('furious rider'; 400 CE).

Philologist Jan de Vries has argued that the Old Norse deities Óðinn and Óðr were probably originally connected (as in the doublet Ullr–Ullinn), with Óðr (*wōðaz) being the elder form and the ultimate source of the name Óðinn (*wōða-naz). He further suggested that the god of rage Óðr–Óðinn stood in opposition to the god of glorious majesty Ullr–Ullinn in a similar manner to the Vedic contrast between Varuna and Mitra.