- A pipe, tube, or channel through which waves are propagated. Also called duct (1). When referring to a pipe or tube with precise dimensions through which microwave energy is transmitted, also called waveguide (2).
- Within the troposphere, a layer which, depending on the temperature and humidity conditions, may act as a waveguide for the transmission of radio waves for extended distances. Also called or duct (3), atmospheric duct, or tropospheric duct.
- A narrow layer that forms under unusual conditions in the atmosphere or ocean, and which serves to propagate radio waves or sound waves. Also called duct (4).