Plain English Meaning
A way of measuring how much greater the pressure is inside the cabin compared to the air outside the aircraft, stated in pounds per square inch.
Full Definition
A unit of pressure measurement expressing the difference in pressure between two points, in pounds per square inch. In pressurized aircraft, psid specifies how much higher the cabin pressure is than the outside ambient pressure at altitude.
Context Anchor
Seen in pressurized aircraft systems, cabin pressure gauges, and aircraft limitations for maximum pressure difference.
Derivation
From psi (pounds per square inch) with a 'd' added for 'differential,' meaning a difference between two pressures rather than an absolute value. The 'd' distinguishes it from psia (absolute) and psig (gauge).