Crypto glossary
Validium
A validium is a scaling system that proves its transactions with validity proofs on Ethereum but keeps the transaction data off-chain to cut costs.
A validium works like a ZK rollup with one key difference: it posts validity proofs to Ethereum, so invalid state transitions are impossible, but it does not publish the underlying transaction data there. Instead, data is held by a data availability committee or an external data availability layer, which signs or attests that it is retrievable.
Skipping on-chain data makes validiums much cheaper and faster than rollups, which is why they were popular for high-volume uses such as games, NFT marketplaces and trading venues built on StarkEx, and why several zkEVM stacks offer validium mode for app-specific chains. Data costs on Ethereum fell with blobs, but the gap still matters for very high throughput.
The trade-off is that users depend on the data holders. If they withhold data, nobody can steal funds thanks to the proofs, but users may be unable to reconstruct their balances and could see funds frozen unless an escape mechanism exists. Rating sites therefore list validiums separately from rollups, and calling one a rollup is a common misdescription.
Example
A blockchain game launches as a validium so players can mint and trade items for fractions of a cent, accepting that a committee of known companies keeps the transaction data.