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.
By definition, the longer-wavelength end of millimeter astronomy overlaps microwave astronomy.