Definition
An instrument that measures the difference between two pressures rather than either pressure on its own. In the airspeed indicator, it compares ram (impact) air pressure from the pitot tube against static air pressure from the static port, and displays the difference as airspeed.
Plain English
A gauge that shows the gap between two pressures. The airspeed indicator uses one to compare the air being rammed into the pitot tube with the still air outside, and turns that gap into a speed reading.
Context Anchor
Seen when studying how the airspeed indicator works and why it needs both pitot pressure and static pressure.
Derivation
Differential comes from the Latin differentia, meaning 'a difference.' The instrument is named for what it does: it reads the difference between two pressure inputs, not the absolute value of either one.
Why Pilots Care
It turns raw pressure differences into the airspeed numbers a pilot uses for safe takeoff, cruise, and landing decisions.
Analogy
It is like a scale that shows the difference between two weights instead of the total weight of one item. The useful information comes from the comparison.
Intuition Check
Do not read differential pressure gauge as simply “a pressure gauge.” The key word is differential: it measures the difference between two pressures.
Example Sentence 1
The airspeed indicator is a differential pressure gauge that compares pitot pressure with static pressure.
Example Sentence 2
During an approach the pilot watches the gauge output to keep airspeed steady in changing wind.