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 it is now known as the Proterozoic (2500–542 Ma ago). Earlier time has most commonly been known simply as the Precambrian, but now comprises the Hadean (4567.17–3800 Ma ago) and Archaean (3800–2500 Ma ago) eons. Hadean is an informal name and the Archaean starting date is not defined stratigraphically. The term ‘Precambrian’ is still in frequent use but is informal.
2. A time unit of 109 present Earth years.