Extremely long periods of time, stretching millions or billions of years; geological time periods.
From Greek 'aion' meaning 'age' or 'eternity', from 'aei' (always). The word entered English via Latin 'aeon'. In geology, it refers to the largest division of geological time. Ancient Greeks used it philosophically to describe eternal, unchanging time.
An 'eon' is so long that the dinosaurs ruled for roughly 165 million years—but that's less than half of one Phanerozoic Eon! We humans have been around for only 0.004% of Earth's 4.5-billion-year existence—basically arrived five minutes before closing time.
Complete word intelligence in one call. Free tier — 50 lookups/day.