Physics Pressure is also the force that is put on a surface with reference to the area of the surface
Pressure conversion rates
1 Atmosphere = 101,325 Pascal
1 Bar = 100,000 Pascal
1 Torr = 1/760 Atmosphere
1 Pound-force per square inch ≈ 6894.757293 Pascal
The force applied perpendicularly to a surface over a determined area is the pressure. The SI unit is the Pascal (so-called after Frenchman Blaise Pascal) which equals one newton per square meter.
The atmosphere was originally to be the pressure of 760mm of mercury at 0 degrees and standard gravity. It equates to the average atmospheric pressure on Earth at sea level (since standardised at 101,325 Pa).
In contrast, the bar is 100,000 Pa. It is therefore only slightly smaller than the atmosphere, and meteorology frequently uses this. However it is usually displayed in millibars.
Whereas the atmosphere is equal to 760mm of mercury, the torr is equivalent to 1mm of mercury. And so, in terms of pascal, the torr = 101325/760 Pa.
The imperial version of the metric Pascal, is the pound per square inch. Uses of the unit are usually more colloquial. Tires, and recreational equipment, are similarly displayed as such. For reference the atmosphere at sea level is roughly 14.7 psi. For example, when pumping up tires, you have to add the atmospheric psi to the value on the gauge in order to determine the absolute value.