Ukko, in Finnish folk religion, the god of thunder, one of the most important deities. The name Ukko is derived from ukkonen, “thunder,” but it also means “old man” and is used as a term of respect. Ukko had his abode at the centre of the heavenly vault, the navel of the sky; hence he was often called Jumala, “Heaven God.” Ukko was believed to control rainfall and thus the fertility of the land, and sacrifices were directed toward him at the beginning of the planting season and in times of drought. As a god of thunder, Ukko has conceptual cognates among most Finno-Ugric peoples, such as the Erzo-Mordvin god Purgine (or Pirgine), whose name is borrowed from the Baltic Perkūnas.