Plural of aeon; extremely long periods of time, or in religious contexts, an age or era of history.
From Greek aion via Latin aeon. The plural form preserves the original sense of discrete ages or eras, which evolved into our modern concept of unfathomably long durations.
The Bible uses 'aeons' to divide history into discrete ages (Dispensationalism), but modern science uses it to mean 'we have no idea how long this is.' Same word, completely different scales of thinking.
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