What is the name of Odin's high seat from which he views all realms?

Odin’s high seat from which he views all realms is called Hliðskjálf.

Hliðskjálf is the elevated seat associated with Odin’s all-seeing authority. From it, he can survey the worlds and observe what happens across them. The name is often interpreted as referring to a high seat or a seat with an expansive view.

The Poetic Edda uses the seat in stories involving both Odin and Frigg. In Grímnismál, they sit there and observe the contrasting lives of the foster brothers Agnarr and Geirröðr. In another myth, Freyr looks from Hliðskjálf toward Jötunheimr and sees the giantess Gerðr, beginning his infatuation with her.

A related name causes occasional confusion. Valaskjálf is described in Snorri Sturluson’s Prose Edda as Odin’s dwelling, with Hliðskjálf inside it. Thus Hliðskjálf is specifically the high seat, while Valaskjálf is the associated hall or residence. It is also entirely distinct from Gjallarhorn, Bifröst, and Valhalla.