Validium is a scalability solution that combines the benefits of zero-knowledge rollups with off-chain storage. It allows for a high throughput of transactions by processing them off the Ethereum chain while only submitting a minimal amount of data back to the main chain for verification. Unlike ZK-rollups, Validium does not store transaction data on-chain, which greatly improves speed and reduces costs, making it ideal for applications requiring privacy and high transaction rates. In Validium, the solution ensures privacy by using validity proofs while keeping the transaction data stored off-chain in a way that's accessible when needed.