Plain meaning
Start with the shortest useful explanation before going deeper.
An Ethereum transaction type introduced in the Dencun upgrade (March 2024) that carries binary large objects (blobs) of temporary off-chain data. Each blob is 128 KB, committed via KZG polynomial commitments, and stored for ~18 days before pruning. Blob transactions created a separate fee market and reduced Layer 2 rollup fees by 10-100x by providing cheap, temporary data availability.