Definition
A unit of pressure expressing the force, in pounds, applied to each square inch of surface area. In aircraft systems, psi is the standard unit used to indicate pressures in oil, fuel, hydraulic, pneumatic, and tire systems.
Plain English
A way of measuring how hard something is pushing on a surface. One psi means one pound of push spread across a one-inch square.
Context Anchor
Seen on oil pressure gauges, tire pressure checks, and aircraft operating limits.
Derivation
Built directly from its component words -- pounds (the unit of force) per (divided by) square inch (the unit of area). The term tells you exactly what it measures: force divided by area.
Why Pilots Care
Correct oil pressure in psi confirms the engine is receiving adequate lubrication to avoid damage from friction and heat.
Analogy
Think of water coming out of a hose. More pressure means the water pushes harder; psi is one way to measure that push.
Grounding Statement
Imagine a one-pound weight resting on a postage stamp. That's roughly one psi.
Intuition Check
Psi does not tell you how much oil is in the engine. It tells you how hard the oil is being pushed through the system.
Example Sentence 1
After engine start, the oil pressure rose into the green arc and stabilized at 60 psi.
Example Sentence 2
Low oil pressure below 25 psi prompted an immediate precautionary landing.