# Data Availability

> The guarantee that the data behind a block can actually be read and reconstructed by anyone.

Canonical URL: https://fudfomo.co/glossary/data-availability
Source: What The Block! Dictionary v1.0 (last updated 2026-04-25), browsable at https://wtb.fudfomo.co.

## Definition

Data availability is the question of whether the contents of a block are actually published somewhere everyone can see. If the data is missing, even a valid-looking proof is not enough, because nobody can check or rebuild the state.

Rollups address this by publishing transaction data back to a layer 1 like Ethereum. Some networks use separate data availability layers to keep costs down while still giving users a way to recover their funds.

## Related terms

- [Rollup](https://fudfomo.co/glossary/rollup): A type of layer 2 that bundles many transactions together and writes a summary back to the main chain.
- [Layer 2](https://fudfomo.co/glossary/layer-2): A faster, cheaper network built on top of a layer 1 like Ethereum. Borrows the main chain's security.
- [Modular Blockchain](https://fudfomo.co/glossary/modular-blockchain): A blockchain design that splits responsibilities across separate, specialised layers.

## See the full catalogue

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