From AMS Glossary
A type of conductor used to carry VHF or microwave energy from one point to another. Most waveguides are hollow (rectangular or circular) and carefully dimensioned according to the frequency and energy to be conducted.
For low-power, short-distance applications a coaxial cable may be used as waveguide. Its primary use in radar is in providing a path for the radio energy between the antenna and the transmitting and receiving systems. Properly shaped open waveguides can themselves be used as radiators (horn antennas) or as feeds at the foci of reflecting antennas.