Plain English Meaning
Push harder on air and you squeeze more of it into the same space, so it gets denser. Ease off the pressure and the air spreads out, so it gets less dense. This only holds true if the temperature stays the same.
Full Definition
The principle that, when temperature is held constant, the density of a gas is directly proportional to its pressure. Increasing the pressure on a fixed volume of air packs more air molecules into that space, raising its density; decreasing the pressure allows the molecules to spread out, lowering its density.
Context Anchor
Seen in aircraft performance discussions, especially when comparing airport elevation, weather conditions, takeoff distance, and climb performance.
Derivation
Pressure comes from a Latin word meaning “to press.” Density comes from a Latin word meaning “thick” or “closely packed.” Together, the phrase points to how strongly air is being pressed and how closely the air is packed in a given space.