n.
Long span of geologic time.
In formal usage, eons are the longest portions of geologic time ( era s are the second-longest). Three eons are recognized: the Phanerozoic Eon (dating from the present back to the beginning of the Cambrian Period), the Proterozoic Eon , and the Archean Eon . Less formally, eon often refers to a span of one billion years.