A stretch of time with a character of its own. Eras are named afterward, by people who can see where they ended.
Latin aera, counters used in reckoning, plural of aes (bronze, money); later, a fixed point from which time itself is reckoned.
Its root is a stack of coins, not a stretch of time: aera named the bronze counters used for reckoning a sum, long before the word reckoned anything as large as an age. An era is still a reckoning at heart, a tally taken only after the counting is done.