Tokoyo (eternal night) (常世)
Tokoyo is also known as Kakuriyo (the world of the dead), and it is the holy precincts which is not changed forever.
(It is a world without any changes, especially a steady world without the law of cause and effect or a world which can be said to have no time base in a certain part.)
It means the world after the death or 'eternity,' and it was also written in Chinese characters as '常夜' (eternal night) in ancient times. The world is an important side of an antinomy in the Japanese Mythology, the Ancient Shinto, and Shinto religion, while the opposite is 'utsushiyo' (land of the living).
Summary
Tokoyo' is a world which is always in the dead of the night, and it is sometimes identified with the world of the dead and yominokuni (realm of the dead) from the meaning of the Chinese characters of tokoyo (常夜).
However, after the release of Shinobu ORIKUCHI's thesis, "Haha ga kuni he/ Tokoyo he" (published in 1920), it does not a mere world of the dead, but a Utopia which is considered to exist over the sea or in the sea when you mentioned especially 'Tokoyo.'
It was defined as "a strange land" where you are given a fortune, knowledge, a life, a long life, and eternal youth and immortality by a visit of marebito (a god which gives people his blessing and leaves).
In the ancient Shinto, it refers to the holy precincts or a world which is different from the real world and located beyond the mountains, the seas, woods, rivers, huge trees, or megalith such as himorogi (a temporarily erected sacred space or "altar" used as a locus of worship), iwakura (dwelling place of a god, usually in reference to a large rock), and so on where 'the aspect of the place' changes.
The Japanese mythology
According to "Kojiki" (The Records of Ancient Matters) and the sixth alternate writing of "Nihonshoki," Sukunabikona, who built up a country along with Okuninushi no mikoto (chief god of Izumo in southern Honshu Island, Japan, and the central character in the important cycle of myths set in that region), went to tokoyo, which was across the seas, after he finished creating the country. It has changed its meaning, and now it is said that tokoyo no kuni means the land of the dead or yomi (world after death).
In "Manyoshu" (Collection of Ten Thousand Leaves), the Palace of the Dragon King where Urashimataro visited was written as tokoyo, where time passed very differently from the real world. From this, it is said to mean a paradise of eternal youth and immortality.
According to an oracle told from Amaterasu Omikami (the Sun Goddess) to Yamatohime no mikoto in "Nihonshoki," Ise is a province where the wave of tokoyo comes in.
The ancient Shinto/the bounds of a sacred place, and kinsokuchi (tabooed land)
A megalith/spiritual rock, shinboku (sacred tree), Chinju-no-Mori (Sacred Shrine Forest), etc. which were considered yorishiro (spiritualistic mediums) in the ancient Shinto, indicate 'border' respectively because '籬' in himorogi (神籬) means 'a fence' and iwakura (磐座) is also written as '磐境' (rock border).
The place beyond the border is considered a sacred area, or tokoyo, and Okinoshima Island is considered sacred as a whole, including Chinju-no-Mori and kinsokuchi. Hedges of evergreen broad-leaved trees in Chinju-no-Mori and shrines are the bounds of a sacred place as well as the border of tokoyo. There are many forbidden places because people are trying not to invite a trouble or disaster caused by various things easily coming and going between utsushiyo and tokoyo.
Doso-shin (traveler's guardian deity) stone statue, small shrines, and jizo (the Guardian Deity of Travelers and Children) located at a roadside leading to a village means not only invocation of a deity for safety travel but also a barrier against tokoyo.
The meaning of tokoyo (eternal night) was assumed to be associated with a border between day and night like evening, and it was called 'Omagadoki' (twilight hour). Midnight, especially a deeply silent night, was considered to be overlapping tokoyo, so the time when something mysterious that did not exist in reality appeared was called 'ushimitsudoki' (the third quarter of the hour of the Ox (approximately 3:00 to 3:30 a.m.)), and people were afraid of the time.
The Ryukyu mythology
The Ryukyu mythology has Niraikanai (paradise across the ocean [folk belief of Okinawa & Amami]) as a similar idea of different world.
Others
One of the Japanese family names. Nagatane TOKOYO, etc.