Hliðskjálf (pronounced roughly “HLITH-skyahlf,” with the “y” being a slide vowel like the “y” sound in “few”) is a high vantage point from which the god Odin (or, on occasion, someone else) can see everything that happens in the cosmos.
There’s some confusion in the primary sources as to whether the name “Hliðskjálf” refers to a general location or to a particular seat or throne in that location. This ambiguity is reflected in the name itself. The second element, skjálf, is an obscure Old Norse word that seems to mean “steep slope,” “crag,” “turret,” or “pinnacle.” The first element, hlið, means “opening” or “gap.” Despite this ambiguity, the characteristics of height and openness are clearly emphasized. Thus, the compound word Hliðskjálf would mean a high place with an expansive view, even if the precise nature of the place is difficult to determine.
Odin is one of the two great masters of the magical art of seidr among the Norse deities (the other being the goddess Freya). Seidr is largely concerned with obtaining enhanced, divinatory perception, a state which Odin achieves while on or at Hliðskjálf. Intriguingly, seidr is often performed from the top of a high, raised platform called a seiðhjallr, which creates the distinct impression that Hliðskjálf is the paradigmatic seiðhjallr, the divine model upon which all such platforms constructed for human use are based.
In any case, Hliðskjálf is part of the rich symbolism through which the ancient Norse depicted their view of Odin’s incredible insight, knowledge, and wisdom.
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 Turville-Petre, E.O.G. 1964. Myth and Religion of the North: The Religion of Ancient Scandinavia. p. 64.
 Price, Neil S. 2002. The Viking Way: Religion and War in Late Iron Age Scandinavia. p. 162.
 Simek, Rudolf. 1993. Dictionary of Northern Mythology. Translated by Angela Hall. p. 322.