Absolute Pressure - denoted as ‘a’ Measured with reference to ZERO pressure, eg. psia

Gauge Pressure - denoted as ‘g’ Measured with reference to ATMOSPHERIC pressure, eg. psig

Thus,

Pressure absolute = Pressure gauge + Pressure atm

Gauge Pressure

"Gauge" pressure is defined relative to atmospheric conditions. In those parts of the world that continue to use English units, gauge pressure is indicated by adding a "g" to the units descriptor. Therefore, the pressure unit "pounds per square inch gauge" is abbreviated psig. When using SI units, it is proper to add "gauge" to the units used, such as "Pa gauge." When pressure is to be measured in absolute units, the reference is full vacuum and the abbreviation for "pounds per square inch absolute" is psia.

Many techniques have been developed for the measurement of pressure and vacuum. Instruments used to measure pressure are called pressure gauges or vacuum gauges

A manometer is a pressure measuring instrument, usually limited to measuring pressures lower than atmospheric. It is often used to refer specifically to liquid column hydrostatic instruments.

A vacuum gauge is used to measure the pressure in a vacuum, which is broadly divided into two categories: high and low vacuum (and sometimes ultra-high vacuum). Many of the different techniques used to measure these categories have an overlap at some point in the pressure range. By combining several different types of gauge it is possible measure system pressure from 10 mbar down to 10e-11 mbar.

Pressure Gauge

Difference in pressure measurements taken at TWO related points; denoted as ‘d’, eg. psid
Delta P = Phigh – Plow

Vacuum - Measured BELOW atmospheric pressure

Definitions of pressure on the Web:

• the force applied to a unit area of surface; measured in pascals (SI unit) or in dynes (cgs unit); "the compressed gas exerts an increased pressure"

• a force that compels; "the public brought pressure to bear on the government"