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eon
1. The largest geologic-time unit, incorporating a number of eras. The equivalent chronostratigraphic unit is the eonothem. Originally, two eons were proposed in 1930 by G. H. Chadwick. The younger was the Phanerozoic Eon (time of evident life), comprising the Cenozoic, Mesozoic, and Palaeozoic Eras, and this term is still used. The term suggested for the preceding eon was the Cryptozoic (time of hidden life). This time has also been called the Archaeozoic (time of most ancient life), but most commonly has been known simply as the Precambrian. Three eons have been proposed for Precambrian time: the Priscoan for time before 4000 Ma ago; the Archaean for 4000–2500 Ma ago; and the Proterozoic for 2500–590 Ma ago; the term ‘Precambrian’ is still in frequent use but is informal.

2. A time unit of 109 present Earth years.

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eon
1. The largest geologic-time unit, incorporating a number of eras. The equivalent chronostratigraphic unit is the (little used) eonothem. Originally, two eons were proposed in 1930 by G. H. Chadwick. The younger was the Phanerozoic Eon (the time of evident life), comprising the Cenozoic, Mesozoic, and Palaeozoic Eras, and this term is still used. The term suggested for the preceding eon was the Cryptozoic (the time of hidden life). This time has also been called the Archaeozoic (the time of most ancient life), but most commonly has been known simply as the Precambrian. Three eons have been proposed for Precambrian time: the Priscoan for time before 4000 Ma ago; the Archaean for 4000–2500 Ma ago; and the Proterozoic for 2500–590 Ma ago; the term ‘Precambrian’ is still in frequent use but is informal.

2. A time unit of 109 present Earth years.

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e·on / ˈēən; ˈēˌän/ (chiefly Brit. also ae·on) • n. (often eons) an indefinite and very long period of time, often a period exaggerated for humorous or rhetorical effect: he reached the crag eons before I arrived. ∎  Astron. & Geol. a unit of time equal to a billion years. ∎ Geol. a major division of geological time, subdivided into eras: the Precambrian eon. ∎  Philos. (in Neoplatonism, Platonism, and Gnosticism) a power existing from eternity; an emanation or phase of the supreme deity.

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eonEritrean, Ghanaian, Himalayan, Malayan, Tigrayan •Actaeon, Aegean, aeon (US eon), Augean, Behan, Cadmean, Caribbean, Carolean, Chaldean, Cyclopean, empyrean, epicurean, European, Fijian, Galilean, Hasmonean, Hebridean, Herculean, Ian, Jacobean, Kampuchean, Laodicean, lien, Linnaean (US Linnean), Maccabean, Mandaean (US Mandean), Medicean, monogenean, Nabataean (US Nabatean), Orphean, paean, paeon, pean, peon, Periclean, piscean, plebeian, Pyrenean, Pythagorean, Sabaean, Sadducean, Sisyphean, skean, Tanzanian, Tennesseean, Terpsichorean, theodicean, Tyrolean

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eon see AEON.