Pressure

The pressure is unitarily defined as the force per unit area, given in the equation below:

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However, the concrete definition for pressure is that its the force perpendicular to the area of a given plane divided by that area, giving a scalar quantity in SI units of N/square meters, or pascals(Pa).

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While this is a great and intuitive definition for some, it may not make a lot of sense without context. So, let's use a conceptual example. The large majority of people get a seasonal flu vaccine every year so they can prevent themselves from sicknesses. When they get this vaccine at their local doctor, they get it in the form of a flu shot, given by an extremely sharp needle. The pressure is the primary reason why the needle is able to penetrate your skin and supply you with vaccines. If this is still confusing, let's compare it to a base example.

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As illustrated below, let's say your friend decides to poke his finger on your shoulder. This would exert roughly the same force on your shoulder as a flu shot would but the finger penetrates your skin, the shot will. This is because the shot has a much lower area of contact with your shoulder than the finger does. Both exert the same force but the needle exerts it on a much lower area which means the needle exerts more pressure on your shoulder than the finger does, causing the needle to be able to break through your skin.

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You may hear the term "atmospheric pressure"(or sea level pressure) used in a given scientific context. All atmospheric pressure is the weight of the atmosphere(usually Earth's) at sea level. Basically, you can think of it like this: Pressure is force/area, and the force exerted by the weight of all of Earth's atmosphere above you divided by the area it acts on you with is the atmospheric pressure on you. At sea level, the atmospheric pressure is 101,325 pascals(Pa).

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The gauge pressure on an object is the pressure of an object in relation to atmospheric pressure. This is useful because sometimes when you measure the pressure of an object, the readings will give you the atmospheric pressure plus the gauge pressure. This isn't really useful and could be inaccurate in specific places. The gauge pressure can also be thought of as the pressure the object itself exerts, outside the factoring in of atmospheric pressure.

Below is a unit table for pressure: