Plain English Meaning

An imaginary level in the atmosphere where the pressure and temperature have agreed-upon standard values. Altitudes can then be measured from that level instead of from constantly changing real-world conditions.

Full Definition

A theoretical reference level where atmospheric pressure equals 29.92 inches of mercury (1013.2 millibars) and the temperature is 15 °C. It is the fixed baseline from which pressure altitude is measured.

Context Anchor

Seen in altimeter, pressure altitude, density altitude, and aircraft performance discussions.

Derivation

‘’ comes from the Latin for ‘something given’ — a fixed everyone agrees to from. ‘Plane’ here means a flat reference level, not an . Together: a given, agreed-upon reference level.