An extremely long period of time, often used in geology to describe the largest divisions of Earth's history, or more generally to mean an age or era.
From Greek 'aion' meaning 'age, lifetime, or eternity.' Entered English in the 17th century, initially borrowed for theological discussions of timelessness and later adopted by geologists to measure truly vast time periods.
When geologists say something happened 'eons ago,' they're literally invoking ancient Greek philosophy about infinity—a word first used to discuss the eternal now measures our most incomprehensible past!
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