A fuzzily defined pressure range of vacuum. The fundamental way most people seem to use the word is as arbitrarily vacuous - having a low enough density for any phenomenon relying on low pressure. Density in vacuum can be measured in either particles per cubic meter or in a pressure measure, such as atmospheres, Pascals, or millimeters of mercury.
For reference, here are some quantitative measures of vacuum:
| low vacuum |
105 - 103 Pascals |
|
| medium vacuum |
103 - 10-1 Pascals |
| high vacuum |
10-1 - 10-4 Pascals |
| very high vacuum |
10-4 - 10-7 Pascals |
| ultra-high vacuum (UHV) |
10-7 - 10-10 Pascals |
| extreme-ultrahigh vacuum (EHV or XHV) |
< 10-10 Pascals |
| space-vacuum |
10-15 Pascals |
| highest man-made vacuum (circa 2002) |
10-10 Pascals |