A hash (in English, Hash) is the result of converting an arbitrary amount of data into a short string of characters of a strictly defined length using a special mathematical function called a hash function. You can feed in anything — text, a file, or a block of transactions — but the output is always a compact fingerprint of the same size, unique to the specific set of input data.

A hash has several important properties that have made it the foundation of cryptography in the blockchain:

• Irreversibility: it is practically impossible to recover the original data from a finished hash.
• Avalanche effect: even the slightest change in the input data completely changes the result.
• Determinism: the same data always produces the same hash.

In cryptocurrencies, hashing is used everywhere: with its help blocks are linked into a continuous chain, the integrity of transactions is verified, and the operation of the Proof-of-Work consensus mechanism is ensured. It is impossible to alter already recorded data unnoticed, because that would immediately change the block's hash and break the entire subsequent chain, which is what makes the blockchain resistant to tampering.