Source : Free On-Line Dictionary of Computing

directional coupler
     
         (tap) A {passive} device used in {cable}
        systems to divide and combine radio frequency signals.  A
        directional coupler has at least three ports: line in, line
        out, and the tap.  The signal passes between line in and line
        out ports with loss referred to as the {insertion loss}.  A
        small portion of the signal power applied to the line in port
        passes to the tap port.  A signal applied to the tap port is
        passed to the line in port less the tap attenuation value.
        The tap signals are isolated from the line out port to prevent
        reflections.  A signal applied to the line out port passes to
        the line in port and is isolated from the tap port.  Some
        devices provide more than one tap output line (multi-taps).
     
        (1995-12-23)