Pounds per square inch, abbreviated to Psi, was a unit of measurement used to measure pressure and/or force/stress. An object under 1 Psi of pressure had the equivalent of 1 lb (0.454 Kg) or force pressing down on each square inch.
Psi was used to measure atmospheric pressure as well as the pressure of shockwaves, such as those released by impacts and sonic weapons.
Common values encountered were 5psi and 20psi. A shockwave of 5psi severely injured most beings, severely damaged vehicles and leveled weaker buildings. A shockwave of 20psi could kill almost all lifeforms outright and completely destroy vehicles. Such a shockwave could also pulverize even the most solid concrete buildings.