A Zero-Knowledge Proof (ZK Proof) is a cryptographic technique that one party can use to prove to another party that it knows something to be true — without revealing any information about what it knows. The ZK Proof could be a statement that a transaction was successfully completed, or that some other information was verified to be correct. The key detail that makes it Zero Knowledge is the fact that it does not reveal the information that it is proving to be true. This has made it possible to process private data without revealing it.
Example: Entity A can prove to Entity B that it knows that X is true without revealing what X contains.
Zero-Knowledge Proofs were introduced in 1989 in the scientific paper ‘The Knowledge Complexity of Interactive Proof Systems’.