A hash is the output of a cryptographic hash function used to verify the authenticity of a particular piece of data.

    The hash function takes input data of any size, computes it, and returns out data of a fixed size.  Though fixed in size, a hash is always unique to any single piece of data – no other data will produce that exact hash, and two different pieces of data will never produce the same hash.

    A hash cannot be reversed – meaning it can never be translated back into the original data.