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