Pressure is defined as force per unit area (on which the force is acting).
or, pressure = force/area
Force is the product of mass and acceleration of an object, as per Newton's second law of motion and has the units of Newton (N). Thus, the units of pressure are Newton per square meter (`N/m^2` ). Another commonly used unit for pressure are psi or pound per square inch.
Also, Pressure = (mass x acceleration)/ area
The pressure exerted by the air is measured in terms of atmosphere or atm. 1 atm is equivalent to the pressure exerted by a column of mercury 76 cm or 760 mm high.
One interesting aspect of the relationship between force and pressure is that as the area reduces, pressure rises for the same applied force. For example, it is impossible for a person to move a wall by exerting force on it. But if we apply the same force on a brick, we can easily move it. If we apply the same force on the tip of needle, think about how much more pressure is developed and what it can accomplish.
Hope this helps.