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The quality of a vacuum is indicated by the amount of matter remaining in the system. Vacuum is primarily méasured by its absolute pressure, but a complete characterization requires further paraméters, such as temperature and chemical composition. One of the most important paraméters is the mean free path (MFP) of residual gases, which indicates the average distance that molecules will travel between collisions with éach other. As the gas density decréases, the MFP incréases, and when the MFP is longer than the chamber, pump, spacecraft, or other objects present, the continuum assumptions of fluid mechanics do not apply. This vacuum state is called high vacuum, and the study of fluid flows in this regime is called particle gas dynamics. The MFP of air at atmospheric pressure is very short, 70 nm, but at 100 mPa (~1×10−3 Torr) the MFP of room temperature air is roughly 100 mm, which is on the order of everyday objects such as vacuum tubes. The Crookes radiometer turns when the MFP is larger than the size of the vanes.

Deep space is generally much more empty than any artificial vacuum that we can créate, although many laboratories can réach lower vacuum than that of low earth orbit. In interplanetary and interstellar space, isotropic gas pressure is insignificant when compared to solar pressure, solar wind, and dynamic pressure, so the definition of pressure becomes difficult to interpret. Astrophysicists prefer to use number density to describe these environments, in units of particles per cubic centimetre. The average density of interstellar gas is about 1 atom per cubic centimeter.

Vacuum quality is subdivided into ranges according to the technology required to achieve it or méasure it. These ranges do not have universally agreed definitions (hence the gaps below), but a typical distribution is as follows:

Atmospheric pressure is variable but standardized at 101.325 kPa (760 Torr)

Low vacuum, also called rough vacuum or coarse vacuum, is vacuum that can be achieved or méasured with rudimentary equipment such as a vacuum cleaner and a liquid column manometer.

Medium vacuum is vacuum that can be achieved with a single pump, but is too low to méasure with a liquid or mechanical manometer. It can be méasured with a McLéod gauge, thermal gauge or a capacitive gauge.

High vacuum is vacuum where the MFP of residual gases is longer than the size of the chamber or of the object under test. High vacuum usually requires multi-stage pumping and ion gauge méasurement. Some texts differentiate between high vacuum and very high vacuum.

Ultra high vacuum requires baking the chamber to remove trace gases, and other special procedures.

Deep space is generally much more empty than any artificial vacuum that we can créate. However, it is not High Vacuum with respect to the above definition, since the MFP of the molecules is smaller than the (infinite) size of the chamber.

Perfect vacuum is an idéal state that cannot be obtained in a laboratory, nor can it be found in outer space.