Wiktionary
submillimeter wave radiation
n. 1 (context astronomy English) Electromagnetic waves with a wavelength less than one millimeter (the band begins at about 0.3 THz). A term often used by radio astronomers. 2 (context engineering English) Electromagnetic waves roughly in the band less than 1 millimeter in wavelength, used as a synonym for T-ray or T-wave 3 terahertz radiation