Hash function
Also called: hash, hashing, hash algorithm
A hash function turns any data into a fixed-size fingerprint that cannot be reversed. It is the basic calculation behind mining.
A hash function takes input data of any size, whether a word, a file or a block header, and produces a fixed-length fingerprint called a hash. With SHA-256, the function Bitcoin uses, this fingerprint is always 256 bits long, or 64 hexadecimal characters, whether you hash a short sentence or an entire encyclopedia.
A good cryptographic hash function has three properties. It is deterministic: the same input always gives the same fingerprint. It is one-way: the input cannot be recovered from the result. And it has an avalanche effect: changing a single character completely changes the fingerprint. This is what makes mining fair: nobody can predict the result, you have to do the calculation to find out.
A machine's hashrate counts precisely how many hashes it computes per second. Bitcoin applies SHA-256 twice in a row to every block header. Other coins have chosen different functions, such as Scrypt, kHeavyHash or Blake3, which is why each one needs a different machine. When a product page states an algorithm, it is really telling you which hash function its chips can compute.
Our page comparing ASICs and graphics cards shows why a specialized chip computes these fingerprints far faster than a general-purpose processor.