Atter is an older germanic term for "poisonous bodily fluid", especially venom of a venomous animal, such as a snake, dragon or other reptile, but also other vile corrupt or morbid substance from the body, such as pus from a sore or wound, as well as bitter substance, such as bile.^[1][2][3] Figuratively, it can also mean moral corruption or corruptness; noxious or corrupt influence, poison to the soul, evil, anger, envy, hatred; as well as destruction and death.^[1][3]

Today, atter is commonly associated with the Norse mythology, where it plays an important role in various contexts – see subsequent section: § In Norse mythology.

Atter is derived from Old English: ātor and ăttor, which in turn derives from Old Saxon: ĕttar, which stems from a Proto-Germanic: *aitrą^[a] (“poison, pus”), which stems from a Proto-Indo-European: *h₂eyd-ro-m,^[a] from a *h₂eyd-^[a] (“to swell; swelling, tumour, abscess”), related to Ancient Greek: οἶδος (“swelling, tumour, abscess, produced by internal action”).^[3][4] It is directly cognate with Old Norse: eitr and its derivatives Icelandic: eitur, Norwegian: eiter, Swedish: etter, Danish: edder, as well as German: Eiter and Dutch: etter, all with similar meaning.^[3] Forms of the word also exist in Scots and a variety of other regional and minority languages in Europe.

While mostly archaic or archaized in English, the word lives on with some strength in other languages. The Icelandic form eitur is the common word for “poison”, while the Swedish form etter is a word for “venom”, as well as the full poetic meaning in dialectal and archaized language.^[3] The German form Eiter and the Dutch form etter are the common word for “pus”.

In Norse mythology, “atter” (Old Norse: eitr) plays an important role in various contexts. In one instance in Gylfaginning, atter is dripped on Loki by a venomous snake placed above him by Skaði. In another, it is blown by the worm Jörmungandr during Ragnarök, leading to the death of Thor.^{[5][6][better source needed][7]} Also in Gylfaginning, atter is described as forming in Ginnungagap, which gave rise to the primordial being Ymir,^[8][9] as described by the jötunn Vafþrúðnir in Vafþrúðnismál:

• Snorra-Edda: Gylfaginning