Definition from Wiktionary, the free dictionary
See also archaean
Archaean (not comparable)
- (geology) of, or relating to the geologic eon from about 3,800 to 2,500 million years ago; comprises the Eoarchean, Paleoarchean, Mesoarchean and Neoarchean eras; marked by an atmosphere with little oxygen, the formation of the first continents and oceans and the emergence of simple life
Proper noun 
- (geology) the Archaean eon.