Hyperboria, or Siberia is a region of Asia located between Greece and Ch'in. It is the homeland of the Northern Amazons and the only known entrance into the Amazon Land of the Dead. Samite nomads also lived in Siberia, specifically in the Northern Steppes.
Xena travelled through Siberia at least twice. The first time, she was with Borias' Army after the attempt to conquer the Lao and Ming kingdoms. She killed the legendary Amazon Queen Cyane and met Alti, who predicted her ability to become the "Destroyer of Nations," and told her of the Ixion Stone.
Over ten years later, Xena returned to Siberia to seek out her soul mate Gabrielle, who she believed was dead, in the Amazon Land of the Dead. Xena helped the Northern Amazons, killing Alti, before learning of Gabrielle's still-living status. It was through Alti in Siberia that Xena received her vision of Gabrielle and her future crucifixion.
When Alti later returned in spirit form into Xena's dreams, she visited Queen Yakut in Siberia in order to discover how to fight back against Alti and protect her unborn child. She traveled with Gabrielle and Amarice, and Amarice joined the Northern Amazons at this time.
After Eve's birth, and in hiding from the Greek Gods, Xena and Gabrielle travelled to Siberian and the Northern Amazons. Although originally intending to ritually have Eve join the Amazon tribe, they decided to stay and briefly reformed the Northern Amazons with the deceased Yakut's help. They later left Siberia to help out Cleopatra in Egypt.
The name of the region was not mentioned onscreen, but can be inferred from official descriptions of it's terrain and climate from "Adventures in the Sin Trade."