What is a Hash?
The output of a cryptographic function that turns any input into a fixed-length string of characters. In Bitcoin, hashing is used to secure transactions, create addresses, and validate blocks.
Why It Matters
Hashing is the cryptographic foundation of Bitcoin's security. A hash function takes any input (transaction, block, number) and produces a fixed-length output that looks random. Change one character of the input. The hash changes completely. Tampering becomes detectable. Bitcoin uses hashing everywhere: addresses (your public key gets hashed to create your Bitcoin address), transactions (every transaction is hashed), and blocks (each block contains a hash of the previous one, creating the "chain"). Bitcoin's entire security model depends on hashing working reliably and being computationally difficult to reverse.
How It Works
A hash function is a mathematical algorithm that converts any input into a fixed-size output string. Bitcoin primarily uses SHA-256. Always 256-bit output regardless of input size. The same input always produces the same hash. Any input change produces a completely different hash (the avalanche effect). It's also computationally impractical to reverse a hash. Bitcoin uses hashing in multiple places: SHA-256 hashing of transactions creates a transaction ID, double SHA-256 hashing is used for blocks, and hashing is part of the mining process where miners search for a hash output below a certain target.