Glossary
The Merge
Ethereum's September 2022 transition from proof-of-work to proof-of-stake, executed by switching execution-layer block production to the existing beacon chain.
The Merge happened on September 15, 2022, at block 15,537,394. Ethereum's existing execution layer (the old PoW chain) stopped producing blocks; from that block onward, blocks were proposed by beacon-chain validators that had been running in parallel since December 2020.
Issuance fell roughly 90% overnight; energy use fell ~99.95%. The Merge was the largest live-protocol transition in crypto history and de-risked the rest of Ethereum's PoS roadmap (withdrawals, EIP-4844, restaking).