Shapella refers to both the Ethereum upgrades: Shanghai and Capella. The Ethereum network consists of two layers: the execution layer and the consensus layer.

The Ethereum Improvement Proposals (EIP-4895) upgrade took part on both layers. The EIP-4895 upgrade will allow validators to perform withdrawals on their staked Ether.

• Execution layer: Shanghai upgrade (named after the Devcon 2 conference host city)
• Consensus layer:  Capella upgrade (named after a star in the Auriga Constellation)

Thus, Shapella is the term used by many crypto enthusiasts to refer to the EIP-4895 upgrade that happened in both layers of the Ethereum network.