Glossary
Epoch
A fixed interval of slots on a PoS chain over which validator duties are scheduled and finality is computed. 32 slots / 6.4 minutes on Ethereum.
On Ethereum's beacon chain, an epoch is 32 slots = 6.4 minutes. Validator committees rotate at epoch boundaries, attestations and proposer assignments are computed per epoch, and finality is reached after two consecutive justified epochs.
The epoch / slot structure exists so that the protocol can issue deterministic schedules in advance, batch slashing and reward calculations, and provide a clean checkpoint cadence. Other PoS chains use similar concepts under different names.