Plain English Meaning
It is simply how much higher the pressure is on one side compared to the other side. You are not measuring either pressure on its own — you are measuring the gap between them.
Full Definition
The difference in pressure between two points in a system, measured by comparing one pressure against another rather than against a vacuum or against the surrounding atmosphere.
Context Anchor
Seen in cabin pressure systems, air-pressure instruments, fuel and hydraulic systems, and maintenance checks where two pressure readings are compared.
Derivation
‘Differential’ comes from the Latin differentia, meaning ‘a difference.’ In this context it signals that the value being measured is the difference between two pressures, not either pressure on its own.
Why Pilots Care
Accurate differential pressure readings are essential for correct and for detecting clogged filters before they cause .