Ukko, sometimes also Ukko ylijumala (Ukko the High God) or Äijä or Äijö (Finnish: male grandparent, old man, also thunder), parallel in Estonian mythology to Uku, is the god of the sky, weather, harvest and thunder in Finnish mythology. The Finnish word for thunder, Ukkonen, is the diminutive form of the name Ukko.[note 1] [note 2] Ukko is often equated with Perkele, and some hold Perkele to be the original personal name of Ukko with the name Ukko being an euphemism. Ukko is held the most significant god of Finnish mythology, although it is disputed by scholars whether this is accountable to later Christian influence.
In the folk poems and prayers he is also given the epithet Ylijumala (English: Overgod), probably in reference to his status as the most highly regarded god and on the other hand his traditional domain in the heavens. Other names for Ukko include Pitkänen (pitkä, "long"), Isäinen (isä, "father"), Isoinen (iso, archaic form of the above, modern meaning "great"). Although portrayed active in myth, Ukko makes all his appearances in legend solely by natural phenomena when appealed to.
According to Haavio, the name Ukko was sometimes used as a common noun or generalised epithet for multiple deities instead of denoting a specific god.
Ukko possessed a weapon, often a hammer called Ukonvasara (English: Ukko's hammer), sometimes also an axe (Finnish: Ukonkirves) or a sword, by which he struck lightning (see thunderbolt). Ukko's weapon was largely comparable to the Norse Mjölnir, and Iron Age emblematic pendants depicting hammers and axes similar or identical to Scandinavian specimens have been unearthed in Finland. Like Mjölnir, Ukko's weapon has been linked by some to the boat-shaped battle axes of the Corded Ware culture.
Thunderbolts were sometimes called Ukon vaaja (English: bolt of Ukko) or Ukon nuoli (English: arrow of Ukko). It is possible that the Birch bark letter no. 292, written in a Baltic-Finnic language and unearthed in Novgorod, makes use of the metaphor, also referring to Ukko as doom-god according to one interpretation. The name Ukon vaaja was also used of Neolithic stone tools such as battles axes, which were often employed as thunderstones to be buried at the corners of dwellings, for example.
Thunderstorms were sometimes interpreted as result of Ukko copulating with his wife Akka ("old woman" in modern Finnish, whereas ukko carries the meaning of "old man"). He also was believed to cause thunderstorms by driving his chariot through the skies.
A viper with a serrated line on its back was considered a symbol of thunder. Neolithic stone carvings have been found in Russian Karelia which have features of both snakes and lightning. It is, however, uncertain whether these are directly connected to the figure of Ukko. Evidence for worship of snakes is found among different cultures around the Baltic, amongst them Estonians and Finns.
There is evidence that the rowan tree was held sacred to Ukko. Rauni, a vaguely defined being has been hypothesised to be cognate to Germanic words for the rowan tree through Old Norse *raunir.
The ladybird was also considered sacred to Ukko and called ukonlehmä (English: Ukko's cow).