Geological time scale
The vast expanse of geological time has been separated into eras, periods, and epochs. The numbers included below refer to the beginnings of the division in which the title appears. The numbers are in millions of years.
Adapted from Lutgens and Tarbuck. They cite the Geological Society of America as the source of the data.
There is another kind of time division used - the "eon". The entire interval of the existence of visible life is called the Phanerozoic eon. The great Precambrian expanse of time is divided into the Proterozoic, Archean, and Hadean eons in order of increasing age.
The names of the eras in the Phanerozoic eon (the eon of visible life) are the Cenozoic ("recent life"), Mesozoic ("middle life") and Paleozoic ("ancient life"). The further subdivision of the eras into 12 "periods" is based on identifiable but less profound changes in life-forms. In the most recent era, the Cenozoic, there is a further subdivision of time into epochs.
Lutgens & Tarbuck
Ch 2, 18