A device installed at an intermediate point on a transmission line that enables new signals to come in and existing signals to go out. In a typical example, most signals pass through the device, but some would be "dropped" by splitting them from the line. Signals originating at that point can be "added" into the line and directed to another destination.
Add/drop multiplexing can be done with optical or electronic signals. The device may deal only with wavelengths, or it may convert between wavelengths and electronic TDM signals. The setup in an add/drop multiplexer is generally static, and the device is not reprogrammed very often. See TDM
, digital cross-connect
Note that the new signal being added can use the same optical channel (wavelength) as the dropped signal. (Illustration assistance courtesy of Jeff Hecht.)