Period of a billion years (4)

I believe the answer is:

aeon

(Other definitions for aeon that I've seen before include "Vast age" , "Immeasurably long period of time" , "Longest division of geological time" , "An immense period of time" , "Eternity" .)

I've seen this clue in The Times.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.