Directional couplers are directional in the sense that the coupled port only couples the signal going from the input port to the output port. This is defined as "coupling." When the return signal goes from output port to input port, the coupled port sees a very low level of that signal. This is defined as "isolation." The directivity is defined as the "isolation" minus "coupling." However, designers should note that directional couplers are bi-directional in the sense that the signal loss from the input port to the coupled port is the same as signal loss from the coupled port to the input port, under the law of reciprocity.