Æsir (Old Norse; singular: áss) or ēse (Old English; singular: ōs) are gods in Germanic paganism. In Old Nordic religion and mythology, the precise meaning of the term "Æsir" is debated, as it can refer either to the gods in general or specifically to one of the main families of gods, in contrast to the Vanir, with whom the Æsir waged war, ultimately leading to a joining of the families. The term can further be applied to local gods that were believed to live in specific features in the landscape – such as fells. The Old English medical text Wið færstice refers to the Ēse, along with elves, as harmful beings that could cause a stabbing pain, although exactly how they were conceived of by the author of the text is unclear.

Áss and its cognate forms feature in many Germanic names, such as Oswald and Ásmundr, and in some place-names in Norway and Sweden. The Æsir further likely give their name to the A-rune, attested in the Elder Futhark, Anglo-Saxon Futhorc and Younger Futhark.

The modern English term "Æsir" is derived from the plural Old Norse term æsir, the singular of which is áss. In Old English, the term used is the cognate ōs (pl. ēse). In continental West Germanic languages, the word is only attested in personal and place names such as Ansila, Ansgeir and Anshram. The cognate term Ansis was recorded as a Latinised form of an original Gothic word by Jordanes, in the 6th century CE work Getica, as a name for euhemerised semi-divine early Gothic rulers. The Old High German is reconstructed as *ans, plural *ensî.

The corresponding feminine form in Old Norse is ásynja (pl.:ásynjur), formed by the addition of the -ynja suffix, denoting a female form. A cognate word for "female áss" is not attested outside Old Norse, and a corresponding West Germanic word would have been separately derived with the feminine suffixes -inī or -injō.

The Proto-Germanic form is typically reconstructed as *ansuz. This form of the word is attested in Proto-Germanic during the Roman Imperial Age as a component of the name of the goddess *Vih-ansa (potentially translating to "Battle-goddess") and probably also in an inscription from around 200 CE on a buckle from Vimose that reads a(n)sau wīja ("I dedicate this to the Æsir"). The word *ansuz is in turn is typically derived from Proto-Indo-European *h₂ems-u-, making the terms closely related to Sanskrit: ásura, Avestan: ahura ("god" or "lord") and Hittite: ḫāši⁠ ("to give birth" or "to beget"). An alternative is that the terms for "Æsir" are related to *ans- ("beam" or "post"), the ancestor of Old Norse: áss ("the main beam of a house"). In this case, the name would likely have originated due to the equation of gods and carved posts in Germanic religion.

Snorri Sturluson and Saxo Grammaticus proposed that the term "Æsir" instead derives from "Ásiamenn ("Asians"), and the idea that the gods originated in Asia, later migrating into Northern Europe. This is however not supported by modern scholars and attributed to medieval scholarship on the matter and an attempt to connect the Scandinavian peoples with Classical antiquity and Christianity rather than a reflection of actual Germanic mythology.

Áss is further found in Old Norse compound nouns such as Asbrú ("Æsir's bridge"), Ásgarð ("home" or "enclosure of the Æsir"), ásmegin ("strength of the Æsir") and the names for Thor Asa-Þórr ("Æsir-Thor") and Asabragr ("Æsir-lord"). The only modern word that is derived from terms for æsir, other than learned borrowings from medieval languages is Swedish: åska, meaning "thunderstorm", which is derived from earlier åsekja ("the driving of the áss"), which would derive from the reconstructed Old East Norse:*ās-ækia. This is likely in reference to the belief in thunderstorms originating from Thor's riding across the skies.

Terms for Æsir form parts of Germanic names in multiple Germanic languages. Examples of this include Old English: Ósbeorn, Óslác, Ósweald (Modern English: Oswald) and their corresponding Old Norse equivalents, Ásbjörn, Áslákr and Ásvaldr. Other examples include Old English: Óswine and Old Norse: Ásmundr. Some Proto-Norse personal names feature *ansuz such as Ansugastiz, Ansugīslaz and *Ansulaibaz.