Submillimeter Astronomy (or Submillimetre Astronomy, depending upon the locality's spelling of "meter/metre") is the branch of astronomy dealing with the observation and analysis the longest Wavelength Infrared radiation from astronomical sources. There is no standard precise definition of the range, but the range 450um to 1mm is clearly part of it. The submillimeter range is used for investigating Molecular Clouds and Star Formation, and star forming galaxies (e.g., Submillimeter Galaxies).
Some submillimeter-capable telescopes:
Far Infrared (FIR)