One way hash
A One way hash is a mathematical way to create a (usually smaller) unique representation of a given set of data. It's sometimes referred to as a "fingerprint". It is called "one way" because it's easy to create the hash value from the input data, it is intended to be impossible to reconstruct the original data from the hash value. SHA and MD5 are popular examples of a one way hash, however be forwarned both have reported weaknesses.