The term "absolute pressure" refers to the sum of the atmospheric pressure and gage pressure, expressed in the equation Pabsolute = Pgage + Patm. Absolute pressure is typically measured against a pure vacuum.
Pressure is defined as the amount of force exerted on a unit of surface area. Pressure is usually measured relative to a standard value. Generally, the standard value of the atmospheric pressure at sea-level is used, known as "standard atmosphere" and equal to 1 atm.
One technique in measuring pressure is by using a manometer, which is an instrument that determines differences in pressure through varying liquid heights. Gage pressure is measured against atmospheric pressure, while absolute pressure is measured against a vacuum by emptying one end of the device.