Epoch

An epoch groups a fixed number of blocks or a fixed span of time into a single unit that a blockchain protocol treats as one administrative cycle. Whatever happens inside that window, whether it is reshuffling who validates transactions, recalculating mining difficulty, or checking whether the chain can be considered final, is settled together rather than block by block.

On Ethereum, which moved to Proof of Stake in September 2022, time is sliced into 12-second slots, and 32 consecutive slots make one epoch, roughly 6.4 minutes. At the start of each epoch the network reshuffles which validators sit on which committee, using the RANDAO randomness beacon so no single actor can predict or manipulate future assignments. The first block of an epoch also serves as a checkpoint: once two consecutive checkpoints gather votes from at least two-thirds of staked ether, the earlier one becomes finalized under the Casper-FFG rule, giving transactions on Ethereum irreversible finality in about two epochs, roughly 12.8 minutes.

Other chains use the word differently. Bitcoin's proof-of-work network runs a 2,016-block "difficulty epoch," roughly two weeks at a 10-minute block time, at the end of which the protocol compares actual mining speed to target and adjusts difficulty up or down, capped at a 4x swing to prevent shocks. Many other Proof of Stake networks apply the same batching idea to distribute staking rewards or rotate block producers.

For users, epochs matter mostly indirectly: they explain why staking rewards post periodically rather than continuously, and why a transaction's confirmation status can jump from "included" to "finalized" in discrete steps rather than gradually.