PSI stands for pounds per square inch, a unit used to measure pressure. One PSI is equal to the amount of force exerted by a pound of matter in Earth's gravity on an area 1 square inch in size.
PSI is commonly used to measure the pressure of gases, such as tire pressure in cars as well as fluid pressure in many industrial settings. As it is a nonmetric unit, its use is less common outside the United States and the U.K. In other countries, the SI unit Pascal is used instead, and 1 PSI is equal to 6,894.76 Pascal.