Archaean (adj.)

"of the earliest geological age," 1872, coined by U.S. geologist and zoologist James Dwight Dana (1813-1895) from Latinized form of Greek arkhaios "ancient," from arkhē "beginning," verbal noun of arkhein "to be the first," hence "to begin" and "to rule" (see archon).

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Definitions of Archaean from WordNet

archaean (adj.)
of or relating to the earliest known rocks formed during the Precambrian Eon;
Synonyms: archean