"Submillimeter wavelengths" refers to a range or type of electromagnetic radiation that has a very short wavelength, smaller than a millimeter. Full definition
Five distant galaxies so choked with dust that they are completely invisible at optical wavelengths have been spotted at submillimetre wavelengths by the European Space Agency's Herschel telescope. (newscientist.com)
This lack of moisture, combined with the thin atmosphere at high altitude, offers ideal conditions for observing the cosmos at the still mysterious millimetre and submillimetre wavelengths which are emitted by cooler objects. (newscientist.com)
The antennas operate at wavelengths between 0.00035 and 10 millimetres: that is, from submillimetre wavelengths to the beginning of the microwave range. (astro-canada.ca)