(1) A System or material designed to confine and direct electromagnetic waves in a direction determined by its physical boundaries. Abbreviated WG. (2) A structure that guides electromagnetic waves along its length.

Alternate definition of waveguide: A material medium that confines and guides a propagating electromagnetic wave. Note 1: 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. Note 2: 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. Note 3: In the RF regime, ionized layers of the stratosphere and refractive surfaces of the troposphere may also act as a waveguide.