(
Message 
Digest 
5)  A popular cryptographic hash function developed by Ronald Rivest (the "R" in RSA) which is used to create a message digest for digital signatures.  MD5 is similar to the previous MD4 method as both were designed for 32-bit computers, but MD5 adds more security since MD4 has been broken.  The earlier MD2 function was designed for 8-bit computers.  See 
cryptographic hash function.