Crypto glossary
Sync committee
A sync committee is a rotating group of 512 Ethereum validators that signs every new block header so light clients and bridges can follow the chain cheaply.
Introduced with the Altair upgrade in 2021, the sync committee is a set of 512 validators chosen at random to serve for 256 epochs, roughly 27 hours. During that period its members sign the head block header in every slot, and their BLS signatures are aggregated into a single compact signature that is included in the next block.
The point is to serve light clients. Instead of tracking hundreds of thousands of validators, a wallet, phone or smart contract only needs to know the current committee, check its aggregate signature and follow the hand-over to the next committee. Trust-minimised bridges and light-client libraries rely on this to verify Ethereum state from other chains or from a browser.
The security assumption is weaker than full finality: a light client trusts that a majority of 512 randomly selected validators is honest, and sync committee signatures are not themselves subject to slashing. Members earn extra rewards while serving, and an offline member loses them, which is why being selected is a noticeable event for small stakers.
Example
A cross-chain light client on another network accepts an Ethereum block header only after verifying that more than two-thirds of the current sync committee signed it.