A one-way function that uniquely represents the input data. Can be thought of as a unique digital fingerprint. The output is a fixed size even though the input can be arbitrarily large. Not encryption because it does not allow recovery of the original message. A popular hashing algorithm is the SHA-256, which returns 256 bits or 64 hexadecimal characters. The Bitcoin blockchain uses the SHA-256. Ethereum uses the Keccak-256. Also known as a hash or message digest.