Definition - What does Vacuum Gauge mean?
A Vacuum Gauge is a pressure measuring instrument that measures pressure in a vacuum. Generally this pressure is usually below atmospheric pressure. These vacuum gauges can be used either in receivers of air pumps or in steam condensers. Vacuum pressure is the additional pressure in any system that is relative to the atmospheric pressure. It is also known as pressure gauge.
Petropedia explains Vacuum Gauge
Vacuum pressure is zero-referenced against ambient air pressure and is equal to the difference between absolute pressure and atmospheric pressure. The negative signs are generally omitted when measuring vacuum pressure and in order to measure system pressure continuously different types of gauges are combined. Most of the tools that measure pressure give the pressure of system with regard to pressure gauge and opposed to absolute pressure. Vacuum pressure is referred to as positive pressure when it is higher than ambient pressure and a vacuum or negative gauge pressure when lower than atmospheric pressure.