Glossary
Hash
The output of a cryptographic hash function — a fixed-length fingerprint of some input data, used to identify blocks, transactions, and addresses.
Informally, a "hash" is the digest produced by running data through a cryptographic hash function. Blockchains use hashes as identifiers: every block has a hash, every transaction has a hash, and addresses are hashes of public keys.
Because the function is one-way, you cannot recover the input from the hash, and because tiny input changes produce wildly different digests, any tampering with the underlying data is immediately visible.