Odds are that you’ve heard of the term PSI, which is one of the most common units of pressure measurement. PSI is used across industries and applications to describe and assess the amount of force being exerted by something. Take, for example, tire pressure PSI. When you pump air into a tire, the molecules will bounce around within the tire, which exerts pressure against the inside; this pressure is the air pressure of your tires. In terms of air compressor measurements, however, PSI has a slightly different meaning.