The term “vacuum,” as it pertains to the use of vacuum to study its effects or to enable something to happen in a high technology sense, has a broad range of definitions and sub-categories.

Two very different definitions are:

1. a space entirely devoid of matter.

2. an enclosed space from which matter, especially air, has been partially removed so that the matter or gas remaining in the space exerts less pressure than the atmosphere.
Using Torr (derived from Evangelista Torricelli, an Italian physicist) as the unit of measure for “vacuum,” to be entirely devoid of matter would seem to mean “0” Torr. But there are no known instances where “O” Torr exists, so to provide some perspective:
Interstellar/Deep Space = ~10-17 Torr
Lunar Surface = ~10-11 Torr
International Space Station = ~
10-9 Torr
>264,000 Feet (80.5 km) = U.S. definition of space flight = ~10-3 Torr
>105,000 Feet (32 km) = turbojets no longer function = ~8 Torr
~35,000 Feet (10.7 km) cargo & passenger airline flights = ~141 Torr
Common definitions of vacuum are:
Low Vacuum = <atmosphere to ~25 Torr
Medium Vacuum = ~25 Torr to 10-3 Torr (also called “rough” vacuum)
High Vacuum = 10-4 Torr to 10-8 Torr
Ultra High Vacuum (UHV) = 10-9 Torr to 10-12 Torr
Extremely High Vacuum (XHV) = <10-12 Torr  
Note the descriptive words are opposite what most people would think in terms of ‘pressure.’ The lower the ‘pressure,’ the higher the vacuum. When you describe a chamber as being at ‘low vacuum,’ you could say it means it is ‘less evacuated’ than a chamber at high vacuum.
The majority of high technology industrial and research related work in vacuum is performed at levels below ~10 Torr. 
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