Eon (noun)

1. An immeasurably long period of time, typically one marked by major geological or astronomical events.
2. An indefinitely long time.

Origin:

Early 19th century: from greek aiōn "age, eternity".

Examples:

1. The Earth has been in existence for eons.
2. The dinosaurs roamed the Earth for millions of eons.
3. The star is estimated to shine for eons to come.
4. The eon of the dinosaurs came to an end with a massive asteroid impact.
5. Geologists study the rock formations that were formed eons ago.