Any algorithm that maps data of variable length to a hash value which serves as a safeguard against accidental corruption and provides a means for efficient searching of the data. Checksums are a very simple example, but more sophisticated hash functions may be one-way functions and so provide some information security.
Computer
Seehashing, hashing algorithm.
Internet
A transformation on data that has the property that the result of the transformation cannot be used to find the original data. Such functions are one of the main pillars of cryptography.