Crypto glossary
Data availability committee (DAC)
A data availability committee is a group of known parties that store a chain's off-chain data and sign that it can be retrieved, used by validiums and some L2s.
When a scaling system does not publish its transaction data on Ethereum, someone has to keep it and promise to share it. A data availability committee (DAC) is a set of named organisations that each store a copy of every batch and sign a certificate saying so. The chain's contract on L1 accepts a batch only if enough members, a set threshold, have signed.
DACs make validiums and hybrid chains much cheaper than rollups that pay for on-chain data. Arbitrum Nova's AnyTrust design uses one, and several zkEVM stacks let chains configure a DAC for validium mode. Some designs add a fallback: if the committee does not sign, the chain switches to posting data on Ethereum.
The security assumption is trust in people, not cryptography. If enough members collude or go offline, users may be unable to rebuild balances and funds can be frozen, though validity proofs still prevent theft. Committees are often small and made up of partners of the project, so their composition and threshold deserve scrutiny.
Example
A validium's DAC has seven members and requires five signatures; when two members suffer outages the chain keeps running, but a third failure would halt new batches.