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The force per unit area that one region of a gas, liquid, or solid exerts on another. Pressure is usually measured in Pascal units, atmospheres, or pounds per square inch. ◇ A substance is said to have negative pressure if some other substance 
exerts more force per unit area on it than viceversa. Its value is simply the negative of the pressure exerted by the other substance. 
Pressure means how much something is pushing on something else. It is expressed as force per unit area: P=F/AIn technology, pressure is often specified in multiples of atmospheric pressure.It can also be defined as the thrust [compressive force acting perpendicularly to the surface of a body] acting per unit area.Since pressure depends on both force and area, the same amount of force applied by a hand will make less pressure than when applied by something smaller, like a finger.Pressure is also related to density. By theideal gas law, for example, doubling the pressure can put twice as much gas in a bottle.To measure the amount of pressure people use these units: