What is a cryptographic hash function?

A function that turns any input data into a fixed-length string (its hash) that acts as a unique, tamper-evident fingerprint of that input.

Feed in a name, a file, or a whole block and you get a constant-size output; change one character and the output changes completely. It is the single most important primitive in a blockchain — used to fingerprint blocks, link them, pose mining puzzles, and derive addresses.

Go deeper:

• Cryptographic hash function — Wikipedia — diagrams the avalanche effect: one changed word, a wholly different digest.