Blob
A large chunk of temporary data attached to an Ethereum block, priced separately from gas and deleted after a few weeks.
Blobs were introduced by EIP-4844 in the Dencun upgrade in 2024 to give rollups a cheap place to publish batch data. Contracts cannot read a blob's contents; the EVM sees only a commitment to it, which is what allows blobs to be priced far below ordinary calldata while still proving that the data was published to the network. Blob space has its own fee market with its own base fee that rises and falls with blob demand independently of the gas base fee, so rollup costs and ordinary transaction costs move separately. Nodes discard blobs after a retention window measured in weeks, because their purpose is to demonstrate publication rather than to serve as permanent storage.
En pratique
Rollups moved their batch data from calldata to blobs after Dencun, which sharply reduced the base-chain cost of posting a batch.
Le malentendu courant
Blob data is not stored on Ethereum permanently; nodes prune it after the retention period, so anyone needing it later must archive it themselves.