A general term for any network device that controls the operation of other network devices. For example, in a point-to-point connection between an IBM controller and a 3270 terminal, the controller is the master station while the terminal is a slave station.
Or, in a multipoint circuit, one of the devices might function as a master station while all the rest are slaves.
Typically, the master station polls all the slave stations regularly and in order, to determine whether a slave wants to initiate communication with the master. (This is in contrast to a peer multipoint circuit such as in an Ethernet network, in which each station can contend for control of the media.)
Another scenario might involve a token-passing network in which one of the devices might function as a master station and be responsible for detecting and replacing lost tokens.