Data availability

Data availability means the raw transaction data behind a blockchain block is actually published so anyone can download and check it, not just a summary or proof that it happened. This matters because if data isn't available, nobody can verify a chain's state or catch fraud, which defeats the point of a public ledger. Rollups highlight the issue well: Optimism and Arbitrum post their transaction data to Ethereum itself, paying for that space, while some cheaper alternatives post data to a separate layer or a committee of nodes instead. If that outside layer goes down or withholds data, users can't reconstruct balances or challenge bad transactions, even if the chain still produces new blocks.