The Free On-line Dictionary of Computing (30 December 2018):

address mask
subnet 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)