An extreme amount of time.
When used by mortals it often refers to their feeling of time, they feel like it is taking an extreme amount of time.
When a god-like being, an eternal, god or immortal being uses it an eon will often refer to truly huge amounts of time. This huge amount of time is often up in millions and billions of years.
This race uses eons to develop!
Eternals are beings of eternity, any being which have existed and will exist for eternity. It can also be a concept within a book or universe, anything that always have existed and always will exist.
Eternals are being defined as any concept or being that has existed and will exist for all of eternity within a certain setting. Death(void), creation(beginning), fate(eternity), chaos(entropy), balance(order)