Pressure is defined as the normal force per unit area and has units of lb/in2 or N/m2.
For a gas the pressure is related to temperature through the ideal gas law.
When the temperature is equal to zero, the pressure must also be equal to zero. When pressure is measured relative to this zero pressure state, it is called the absolute pressure. Pressure is also frequently specified relative to the pressure of air in the surroundings. This measure of pressure is called gage pressure.
This definition of pressure is often referred to as the static pressure. In fluid dynamics there are other “psuedo-pressures” encountered. Some of these include stagnation pressure and total pressure. When computing state variables in fluids it is important that the static pressure be used.
Understand the differences between gage, absolute, and atmospheric pressure