Hash / Hash Function
Plain English
A mathematical function that converts any data into a unique fixed-length code (like a fingerprint). Change even one letter in the input and you get a completely different hash. This is how blockchains detect if anyone tried to change old data.
Technical
Cryptographic one-way function that maps arbitrary-length input to fixed-length output (hash/digest). Properties: deterministic (same input = same output), avalanche effect (small input change = completely different output), preimage resistance (can't reverse), collision resistance (can't find two inputs with same output). Blockchains use SHA-256, Keccak-256, etc.