Source : Free On-Line Dictionary of Computing

Keyed-Hashing Message Authentication
     
         (HMAC) A mechanism for message
        {authentication} using cryptographic {hash functions}.  HMAC
        can be used with any iterative cryptographic hash function,
        e.g., {MD5}, {SHA-1}, in combination with a secret shared key.
        The cryptographic strength of HMAC depends on the properties
        of the underlying hash function.
     
        [RFC 2104].
     
        (1997-05-10)