Pressure is a continuous force something exerts on something else, such as air in a tire. In the example of a tire, there is a certain amount of space available inside the tire and wheel, and when filled with air at atmospheric pressure, it would be measured at 0 PSI. This is a relative amount, because it is compared to atmospheric pressure. If you start addng air beyond this, the air has no other space to fill, so it becomes compressed, and that compressed air exerts pressure on all surfaces within the wheel and tire combination. That force is what is observed when you put a tire pressure gauge on the valve stem.
PSI: Pounds per Square Inch
If you had a cube with each side being one inch by one inch (one square inch), placing a one-pound weight on it, covering the whole square inch surface, would exert one pound of force on that area of one square inch, which could also be considered one pound per square inch. In the same manner, if you set that one-pound weight on two of those blocks (side by side, not on top of each other), covering the surface area of both cubes, that would be the equivalent of half a pound per square inch; one cube is holding half a pound, and the other cube is holding the other half pound.
Pascal: Standardized Unit of Measurement
Pressure is often measured in Pascals, which is recognized as part of the International System of Units. It is defined as one Newton of force per square meter.
Bar: Common Automotive Unit of Pressure
One bar is equal to 100,000 Pascals, but is not recognized by the International System of Units. One bar is also very close to atmospheric pressure at sea level.
"Pressure" by Muse
The music video for "Pressure" by Muse, which is track three on the album Simulation Theory, and the band's fifth single from the album. The video was produced by Jason Baum via WhiteList, directed by Lance Drake, edited by Jeremiah Mayhew. It also features Terry Crews, Chance Michael and Julia Robinson.