Hash
A fixed-length fingerprint produced by running data through a one-way function; any change yields a completely different output.
A hash function takes any input and returns a short, fixed-size string. The same input always gives the same hash, but flip a single character and the result looks entirely unrelated. You can't run it backward to recover the input.
Blockchains lean on this everywhere. Each block stores the hash of the one before it, so altering old data would change every hash that follows and expose the tampering instantly. Bitcoin uses the SHA-256 algorithm.
Related terms
The math of scrambling and verifying information so only intended parties can read or trust it.
BlockA batch of transactions bundled together, time-stamped, and cryptographically linked to the block before it.
Merkle TreeA structure that hashes transactions in pairs up to a single root, letting you verify one without all.
NonceA throwaway number miners change over and over while hunting for a hash that meets the target.