Vacuum terminology
Vacuum technology - Introduction

From depression to extreme vacuum, users of vacuum techniques each have their own vocabulary, which can sometimes be confusing for the uninitiated.

Vacuum means absence of matter, and by extension and commonly, absence of gas, or at least rarefied gas. In a closed, sealed volume, the number of molecules present at a given moment must be less than the number of molecules in the same volume when open to atmospheric pressure. This definition is inadequate, as it does not allow us to quantify the magnitude of the phenomenon by measurement. As it is difficult to count the molecules, we have chosen a physical quantity proportional to this number of molecules: pressure.

The law of perfect gases gives the pressure :...