A Zero-Knowledge Succinct Non-Interactive Argument of Knowledge (zk-SNARK) is a type of cryptographic proof used to ensure privacy on distributed ledger systems based on blockchain technology. It works by using a secret key before the transaction is broadcast to prove that one party has specific data without actually revealing the data to the network. zk-SNARKS rose to prominence alongside Zcash, Monero, and other privacy-focused blockchain protocols.