The Intensity of Pressure at any point in a Fluid is the same in all direction,when it is at rest.
It is established that the Air possesses some Weight and due to its Weight,Air must exert some Pressure on the Surface of the Earth.The Density of Air is different at different heights due to its property of Compressibility.
The Density of Air has also vary from time to time due to change in its Temperature and Humidity.Due to these factors,the Atmospheric Pressure of Air can not be calculated,as is done in case of Liquid.
The Atmospheric Pressure at Sea Level is 1.03 kilogram/centimeter square.It can also be expressed as 10.3 meters of water,in terms of equivalent Water Column or 76 centimeter of Mercury in terms of equivalent Mercury Column.
It is the Pressure measured by Pressure Measuring Instruments.The Atmospheric Pressure on the Gauge Scale is marked as Zero.Actually Gauge Pressure is always above the Atmospheric Pressure.
It is defined as the sum of Atmospheric Pressure and Gauge Pressure.In the case of Vacuums,the Gauge Pressure is Minus,then the Absolute Pressure will be,Atmospheric Pressure minus Gauge Pressure.
If the Pressure Intensity at a point is more than the Local Atmospheric Pressure,the difference of these two Pressure Intensity is called "Positive Gauge Pressure".
However,If the Pressure intensity at a point is less than the Local Atmospheric Pressure,the difference of these two Pressure Intensity is called "Negative Gauge Pressure or Vacuum Pressure".