Hash / Hash Function
A fixed-length fingerprint of arbitrary data, used throughout blockchains to link blocks and verify integrity.
A cryptographic hash function takes any input (a block of data, a transaction, a file) and produces a fixed-length output — the hash or digest — that appears random. The same input always produces the same output, but even a single character change produces a completely different hash.
Blockchains use hashes to link blocks (each block contains the hash of the previous block), identify transactions (a transaction ID is its hash), and secure the PoW puzzle. Common algorithms include SHA-256 (Bitcoin) and Keccak-256 (Ethereum).
Hash functions are one-way: you cannot reverse a hash to recover the original input. This property is fundamental to the security of digital signatures, password storage, and blockchain integrity.