PSI stands for "pounds per square inch" and is a unit of measurement for pressure or stress based on avoirdupois units. It is the pressure resulting from a force with magnitude of one pound-force applied to an area of one square inch. PSI is commonly used to measure the pressure of gases, known as pneumatic pressure, or liquids, also called hydraulic pressure. It is also used to measure tensile strength and elastic modulus strength, which controls the stiffness of materials. PSI is most commonly associated with tire pressure, and gauges measuring pressure are used in all manner of instrumentation for vehicles, pneumatic and hydraulic machines, and industrial and safety systems. In the USA, PSI is the primary unit of measure for pressure, and almost all pressure instruments are specified and display in pounds per square inch.