Question: what is pressure ? and three different units for pressure ?

Jean Bourke answered on 22 Nov 2012:

Pressure is defined as force per unit area. It’s how much something is pushing something else.

The units are based on what it is: force is measures in Newtons and area is measured in meters squared, so pressure is Newtons per meter squared. We give this a much simpler name: Pascals.

There are other units for pressure mainly based on our atmosphere. 1atmosphere is the pressure due to the atmosphere pressing down on us measured at sea level. 1 atmosphere is about the same as 1 x 10^5 Pascals.

You can also use different units for the area such as square feet or square inches and you can used pounds to describe the force as weight is a force due to gravity. So you can have pounds per square inch or pounds per square foot. So long as it is based on force per unit area it will be measure of pressure.