What is the category name for the largest division of time used on the geologic time scale?

Eon

Explanation:

The geological past is divided into several different categories of time units, some of which are very long, while some much shorter. The division is made based on marking events that happened, thus ending one geological time unit, and starting another, new one. The largest division of time used is the eon. The eon lasts the longest of all time unites, spreading for more than half a billion years, which is actually the shortest one, up to couple of billion years. There have been only four eons so far. Hadean, Archean, Proterozoic, and Phanerozoic. The last one, the Phanerozoic started around 541 million years ago and it still lasts, thus this eon has seen the rise of multiple organisms that dominate the planet, as well as their downfall, most noticeably the arthropods, sinapsids, dinosaurs, mammals, and lastly the humans as part of the mammals and the most intelligent animal to have ever lived on the planet.

brave new world, aldous huxley (1932)