An Ethereum blockchain upgrade in April 2023 that required all participants to adhere to the new proof-of-stake (PoS) concensus method. It also allowed the first proof-of-stake participants to withdraw their ether over a period of time.
Shanghai + Capella = Shapella
At the programming level, Shanghai deals with the execution layer, and Capella deals with the logic of the consensus mechanism, thus Shapella is an insider term for Shanghai. See
Ethereum staking and
Ethereum.