In Old Norse, áss (or ǫ́ss, ás, plural æsir; feminine ásynja, plural ásynjur) is the term denoting a member of the principal pantheon in the indigenous European religion known as Norse paganism. This pantheon includes Odin, Frigg, Thor, Baldr and Tyr. The second pantheon comprises the Vanir. In Norse mythology, the two pantheons wage the Æsir-Vanir War‎, which results in a unified pantheon.

The cognate term in Old English is ōs (plural ēse) denoting a deity in Anglo-Saxon paganism. The Old High German is ans, plural ensî. The Gothic language had ans- (based only on Jordanes who glossed anses with uncertain meaning, possibly 'demi-god' and presumably a Latinized form of actual plural *anseis). The reconstructed Proto-Germanic form is *ansuz (plural *ansiwiz). The a-rune ᚫ was named after the æsir.

Unlike the Old English word god (and Old Norse goð), the term ōs (áss) was never adopted into Christian use and survived only in a secularized meaning of "pole, beam, stave, hill" or "yoke".

Æsir is the plural of áss, óss "god" (gen. āsir) which is attested in other Germanic languages, eg., Old English ōs (gen. pl. ēsa) and Gothic (as reported by Jordanes) anses "half-gods". These all stem from Proto-Germanic *ansis ~ ansuz, which itself comes from Proto-Indo-European *h₂énsus (gen. h₂n̥sóus) "life force" (cf. Avestan aŋhū "lord; lifetime", ahura "godhood", Sanskrit ásu "life force", ásura "god" (< *h₂n̥suró)). It is widely accepted that this word is further related to *h₂ens- "to engender" (cf. Hittite hass- "to procreate, give birth", Tocharian B ās- "to produce").