Microwaves, extremely short radio waves. The term is usually used to refer to radio waves with a wavelength between 30 centimeters and 3 millimeters—that is, with a frequency between 1 gigahertz (billion hertz) and 100 gigahertz. Radio waves as long as 1 meter and as short as 1 millimeter are also sometimes considered microwaves.

An important use of microwaves is in transmitting signals for telephone, television, and other communications systems over a long distance—particularly between communications satellites and the earth's surface. Microwaves are also used in many kinds of radar; in some navigation systems to guide landing aircraft; and in microwave ovens to heat food and other items rapidly.

Microwaves are generated using either semiconductor devices or such electron tubes as the magnetron and the klystron. For transmission over a short distance, microwaves are sent through a coaxial cable or through a hollow metal tube called a waveguide. For long distances, microwaves are transmitted by a parabolic or horn-reflector antenna that is usually designed to direct the microwaves in a relatively narrow beam.

The first practical microwave equipment was built in the 1930's. Its development was spurred during World War II by research for improving radar.