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's called "One Way" because while it's easy to create the hash value from the input data, it is, for all intents and purposes, impossible to reconstruct the original data from the hash value. MD5 is perhaps the most well-known One Way Hash. SHA, the Secure Hashing Algorithm, is widely used today to replace MD5.