What defines a perfect vacuum?

A perfect vacuum is defined as a condition where there is no matter present, which means the pressure is at absolute zero. In practical terms, on Earth, this condition is often represented as 0 psia (pounds per square inch absolute) because this is the absolute pressure reading when there is no atmospheric pressure acting on a space.

Additionally, -14.7 psig (pounds per square inch gauge) correlates with atmospheric pressure at sea level, meaning that if you measure 0 psia, it would feel like -14.7 psig when using gauge pressure (which subtracts the atmospheric pressure). Since both 0 psia and -14.7 psig accurately represent the ideal condition of a perfect vacuum, indicating the same concept of absence of pressure, the answer confirming that both statements are correct validates the definition of a perfect vacuum thoroughly.

6.9 KPa also reflects atmospheric pressure but is not a direct indication of a perfect vacuum, and while it is valid in certain contexts, it does not convey the same universal understanding as the other two measurements, which are directly related to absolute pressure definitions.