From Glossary of Meteorology
Electromagnetic radiation having wavelengths between approximately 1 mm and 1 m (corresponding to 0.3- and 300-GHz frequency) bounded on the short-wavelength side by far infrared (< 1 mm) and on the long-wavelength side by very high frequency radio waves (> 1 m).
Passive systems operating at these wavelengths are sometimes called passive microwave systems. Active systems operating at these wavelengths are called radar, although the definition of radar requires a capability to measure distance that is not always included in active microwave systems. The limits of the microwave region are not precisely fixed.