An eon is a really, really, super-long, impossible-to-measure length of time. If you sit down to dinner hanging your head and moaning that it's been an eon since you ate anything, you’re exaggerating. Four hours ago is not an eon.
Eon goes back to the Greek aiōn, "age." An age is not easy to measure, and neither is an eon. Both are just really long periods of time, but in science an eon is about a billion years. You can use the noun eon for anything that takes a long while, including how long it will be before another planet collides with Earth or how many days until summer vacation starts — one is an eon, the other feels like an eon, or forever.
Primary Meanings of eon
the longest division of geological time
(Gnosticism) a divine power or nature emanating from the Supreme Being and playing various roles in the operation of the universe
n the longest division of geological time
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the period from about 5,400 million years ago until the present
the eon following the Hadean time and preceding the Phanerozoic eon; from about 3,800 million years ago until 544 million years ago
from 2,500 to 544 million years ago; bacteria and fungi; primitive multicellular organisms
the time from 3,800 million years to 2,500 million years ago; earth's crust formed; unicellular organisms are earliest forms of life
the earliest eon in the history of the Earth from the first accretion of planetary material (around 4,600 million years ago) until the date of the oldest known rocks (about 3,800 million years ago); no evidence of life