The ancient Greeks were sometimes called sons of Boreas (north wind). Hyperborea is the land beyond or above the north.
From The Secret History of the World (p. 283-286):
Diodorus Siculus, writing in the first century BC, gives us a description of Britain based, in part, on the voyage of Pytheas of Massilia, who sailed around Britain in 300 BC.
As for the inhabitants, they are simple and far removed from the shrewdness and vice which characterize our day. Their way of living is modest, since they are well clear of the luxury which is begotten of wealth. The island is also thickly populated and its climate is extremely cold, as one would expect, since it actually lies under the Great Bear. It is held by many kings and potentates, who for the most part live at peace among themselves.
Diodorus then tells a fascinating story about the Hyperboreans that was obviously of legendary character already when he was writing:
Of those who have written about the ancient myths, Hecateus and certain others say that in the regions beyond the land of the Celts (Gaul) there lies in the ocean an island no smaller than Sicily. This island, the account continues, is situated in the north, and is inhabited by the Hyperboreans, who are called by that name because their home is beyond the point whence the north wind blows; and the land is both fertile and productive of every crop, and since it has an unusually temperate climate it produces two harvests each year.
Now, it seems that there is little doubt that Diodorus is describing the same location, but we notice that the climate is so vastly different in the two descriptions that we can hardly make the connection. However, let us just suppose that his description of Britain was based on the climate that prevailed at the time he was writing, and the legendary description of the Hyperboreans was based on a previous climatic condition that was preserved in the story. Diodorus stresses that he is recounting something very ancient as he goes on to say:
The Hyperboreans also have a language, we are informed, which is peculiar to them, and are most friendly disposed towards the Greeks, and especially towards the Athenians and the Delians, who have inherited this goodwill from most ancient times. The myth also relates that certain Greeks visited the Hyperboreans and left behind them costly votive offerings bearing inscriptions in Greek letters. And in the same way Abaris, a Hyperborean, came to Greece in ancient times and renewed the goodwill and kinship of his people to the Delians.
Diodorus remark about the relations between the Hyperboreans and the Athenians triggers in our minds the memory of the statement of Plato that the Atlanteans were at war with the Athenians, and we wonder if the Hyperboreans are the real “early Athenians.” After all, the Greeks are said to be “Sons of the North Wind,” Boreas. The relationship of the Hyperboreans to the Delians is expounded upon by Herodotus:
Certain sacred offerings wrapped up in wheat straw come from the Hyperboreans into Scythia, whence they are taken over by the neighbouring peoples in succession until they get as far west as the Adriatic: from there they are sent south, and the first Greeks to receive them are the Dodonaeans. Then, continuing southward, they reach the Malian gulf, cross to Euboea, and are passed on from town to town as far as Carystus. Then they skip Andros, the Carystians take them to Tenos, and the Tenians to Delos. That is how these things are said to reach Delos at the present time.
The legendary connection between the Hyperboreans and the Delians leads us to another interesting remark of Herodotus who tells us that Leto, the mother of Apollo, was born on the island of the Hyperboreans. That there was regular contact between the Greeks and the Hyperboreans over many centuries does not seem to be in doubt. The Hyperboreans were said to have introduced the Greeks to the worship of Apollo, but it is just as likely that the relationship goes much further back. Yes, this is contrary to the idea that culture flowed from south to north, but we are writing a contrary book, so don’t let that bother you! Herodotus has another interesting thing to say about the Hyperboreans and their sending of sacred offerings to Delos:
On the first occasion they were sent in charge of two girls, whose names the Delians say were Hyperoche and Laodice. To protect the girls on the journey, the Hyperboreans sent five men to accompany them … The two Hyperborean girls died in Delos, and the boys and girls of the island still cut their hair as a sign of mourning for them… There is also a Delphic story that before the time of Hyperoche and Laodice, two other Hyperborean girls, Arge and Opis, came to Delos by the same route. …Arge and Opis came to the island at the same time as Apollo and Artemis…
[…] And there is also on the island [Hyperborea] both a magnificent sacred precinct of Apollo and a notable temple which is adorned with many votive offerings and is spherical in shape. Furthermore, a city is there which is sacred to this god, and the majority of its inhabitants are players on the cithara; and these continually play on this instrument in the temple and sing hymns of praise to the god, glorifying his deeds… They say also that the moon, as viewed from this island, appears to be but a little distance from the earth and to have upon it prominences, like those of the earth, which are visible to the eye. The account is also given that the god visits the island every nineteen years, the period in which the return of the stars to the same place in the heavens is accomplished, and for this reason the nineteen year period is called by the Greeks the “year of Meton.” At the time of this appearance of the god he both plays on the cithara and dances continuously the night through from the vernal equinox until the rising of the Pleiades, expressing in this manner his delight in his successes. And the kings of this city and the supervisors of the sacred precinct are called Boreades, since they are descendants of Boreas, “and the succession to these positions is always kept in their family.”
The Secret History of the World further connects the ambassadors of Hyperborea to the legend of the Minotaur, where these would have been sacrificed to the beast in the labyrinth.
Modern occult literature is full of various claims about Hyperborea representing this or that. From the accounts of ancient historians it seems reasonable to associate Hyperborea with Britain or Ireland and the Hyperboreans with the megalithic cultures that were known to exist there during and before the Greek antiquity.