Vacuum is defined as pressure that is?

Vacuum is defined as pressure that is below atmospheric pressure. This means that a vacuum exists when the pressure inside a particular space is less than the surrounding atmospheric pressure, which typically exists at sea level, around 101.3 kilopascals (kPa).

In practical applications, creating a vacuum involves removing air or other gases from a sealed volume so that the pressure drops below this standard atmospheric pressure. This can be important in many scientific and industrial processes, such as in vacuum packing, vacuum filtration, and various manufacturing processes where controlling the environmental pressure can affect outcomes.

The other definitions provided do not accurately represent a vacuum. For instance, if the pressure were at or above atmospheric pressure, it would not constitute a vacuum, and being exactly at atmospheric pressure does not allow for any vacuum conditions. Additionally, defining vacuum as unmeasurable contradicts the principles of vacuum measurement techniques that allow us to quantify the degree of vacuum achieved.