Pressure

Pressures, like temperatures, can be expressed by either absolute or relative scales. Pressure is defined as "normal force per unit area." Figure 1.5 shows a cylinder of water. Pressure is exerted on the top of the water by the atmosphere, and on the
bottom of the cylinder by the water. If a hole were drilled in the side of the cylinder, you would have to exert a force against the water surface area by means of a plug to prevent the water from exiting the cylinder through the hole.

Since absolute pressure is based on a complete vacuum, a fixed reference point which is unchanged regardless of location or temperature or weather or other factors, absolute pressure then establishes a precise, invariable value which can be readily identified. Thus, the zero point for an absolute pressure scale corresponds to a perfect vacuum, whereas the zero point for a relative pressure scale usually corresponds to the pressure of the air which surrounds us at all times, and as you know, varies slightly.