The term millimeter astronomy (or millimeter wave astronomy) indicates the astronomy of observing EMR wavelengths in the general region of a few millimeters (millimeter waves), perhaps including substantial fractions of a millimeter. Sometimes the term is used virtually as a synonym for submillimeter astronomy. Many telescopes span both these ranges, and the choice of term incorporated in the telescope's name seems to be a matter of preference and the phrase millimeter/submillimeter astronomy is sometimes used as more descriptive. By definition, millimeter astronomy overlaps microwave astronomy.
Cosmic microwave background studies fall in this class. Portions of this wavelength-range have the advantage of falling in atmospheric windows allowing observation from the ground, and high altitudes and dry air are advantageous. It has an advantage over longer-wave radio astronomy in that the angular resolution produced by interferometry depends upon the wavelength, shorter being better resolution, and millimeter/submillimeter interferometry such as that of ALMA are producing images with very fine detail.