Crossword Genius

Billion-year span (3)

Ross

I believe the answer is:

eon

'billion-year span' is the definition.
(I've seen this in another clue)

This is the entire clue.

(Other definitions for eon that I've seen before include "Very long period (US and technical spelling)" , "Very long age" , "Huge time period" , "Major division of geological time" , "Age, epoch" .)

I've seen this clue in the Penny Press, the L.A. Times Daily, the Universal and The Washington Post.
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