1. In a communications system, an adapter, i.e., a hardware device, that connects a functional unit, such as data terminal equipment, to a transmission line. See integrated communications adapter. See also communications system,  connection,  data terminal equipment,  device,  functional unit,  hardware,  network interface device,  transmission line. 2. An adapter that (a) consists of (i) necessary circuitry, such as a printed circuit board or an expansion board, and (ii) associated software and (b) enables a device, such as a data processor or a communications controller, to be connected to a communications network or to a computer network. See common communications adapter,  X.21 communications adapter. See also adapter,  circuitry,  communications controller,  communications network,  computer network,  controller,  data processor,  device,  expansion board,  printed circuit,  printed circuit board,  processor,  software.