In the microwave regime, a waveguide normally consists of a hollow metallic conductor, usually rectangular, elliptical, or circular in cross section. This type of waveguide may, under certain conditions, contain a solid or gaseous dielectric material.
In the optical regime, a waveguide used as a long transmission line consists of a solid dielectric filament (optical fiber), usually circular in cross section. In integrated optical circuits an optical waveguide may consist of a thin dielectric film.
In the radio frequency regime, ionized layers of the stratosphere and refractive surfaces of the troposphere may also act as a waveguide.
Source: from Federal Standard 1037C and from MIL-STD-188
Waveguide propagation modes depend on the operating wavelength and polarization and the shape and size of the guide. In hollow metallic waveguides, the fundamental modes are the transverse electric TE1,0 mode for rectangular and TE1,1 for circular waveguides.
See also: Glossary of antenna terms, Radio wave, Atmospheric waveguide, Dielectric waveguide, Closed waveguide, Cutoff wavelength, Propagation , Radiation mode, Feed horn, Angular misalignment loss, Power transmission Dielectric constant, Leaky mode, Filled cable, Glossary of fiber optic terms, Glossary of telecommunications transmission terms, Horn, Transmission medium