Source : Free On-Line Dictionary of Computing

address mask
     
         (Or "subnet mask") A {bit mask} used to identify
        which bits in an {IP address} correspond to the {network
        address} and {subnet} portions of the address.  This mask is
        often referred to as the subnet mask because the network
        portion of the address can be determined by the {class}
        inherent in an IP address.  The address mask has ones in
        positions corresponding to the network and subnet numbers and
        zeros in the host number positions.
     
        (1996-03-21)