Hash
A fixed-length fingerprint produced from any input by a one-way cryptographic function.
A hash function takes data of any size and returns a fixed-length string. The same input always produces the same output, a single changed character produces a completely different output, and there is no practical way to work backwards from the output to the input.
Blockchains use hashes to link blocks, to identify transactions and to prove that mining work was actually performed. Bitcoin uses SHA-256; Ethereum uses Keccak-256.