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 (hā-dē′ən, hā′dē-)
Of, relating to, or being the period of time during which Earth formed, from the start of the solar system (4.6 billion years ago) until accretion, impact, and cooling led to stable Earth-Moon orbits and the formation of the earliest rocks on Earth (3.8 billion years ago). See Table at geologic time.

[After Hades, (from the intense heat during part of this period).]
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Noun1.Hadean - the earliest eon in the history of the Earth from the first accretion of planetary material (around 4,600 million years ago) until the date of the oldest known rocks (about 3,800 million years ago); no evidence of life
Adj.1.Hadean - of or relating to or characteristic of Hades or Tartarus
infernal - being of the underworld; "infernal regions"