Glossary
Data Availability
The guarantee that the data behind a block is published in full to the network, so anyone can reconstruct state — and so rollups can be challenged.
A rollup's security depends on being able to challenge a fraudulent state transition. That challenge needs the underlying transaction data to be available — if the sequencer can hide it, no one can prove anything is wrong.
"DA" is the property of "this data has been published." Layer-1s and dedicated DA layers (EigenDA, Celestia, Avail) compete on the cost and guarantees they offer. Cheaper DA directly translates into cheaper L2 fees, which is why EIP-4844 had such an immediate impact.