The term Millimeter Astronomy (or Millimeter Wave Astronomy) is naturally used for astronomy at Wavelengths in the region of a millimeter.
Sometimes the term is used virtually as a synonym for Submillimeter Astronomy, which even in its strictest interpretation, approaches a millimeter. The fact that telescopes now commonly cover a range encompassing both probably encourages this, and the choice of term incorporated in the telescope's name seems to be a matter of preference. The phrase Millimeter/Submillimeter Astronomy is sometimes used to resolve any ambiguity.
Max Planck Institute for Radio Astronomy (MPIR)