Source : Free On-Line Dictionary of Computing

ACK
     
        1.  /ak/ The {mnemonic} for the ACKnowledge
        character, {ASCII} code 6.
     
        2.  A message transmitted to indicate that
        some data has been received correctly.  Typically, if the
        sender does not receive the ACK message after some
        predetermined time, or receives a {NAK}, the original data
        will be sent again.
     
        [{Jargon File}]
     
        (1997-01-07)