An Acronym for Message Digest 5.
It is one of a family of CryptographicHash Algorithms, and produces a 128-bit digest (or "fingerprint") from arbitrarily long input data streams. It is based on MD2, but is faster. It is 
nowadays considered weak. Do not use it in applications where you might need to guard against malicious modification of your data.
8 pages link to MD5:
RFC: