Definition
The international standard sea-level atmospheric pressure value, equal to 1013.2 millibars (also expressed as 29.92 inches of mercury or 1013.2 hectopascals). It is the reference pressure used in the International Standard Atmosphere (ISA) for calibrating altimeters, calculating performance, and comparing weather observations.
Plain English
It is the agreed-upon "normal" air pressure at sea level. Whenever you see this number, it is the baseline that the rest of aviation measures pressure against.
Context Anchor
Seen in aircraft performance, weather, and altitude discussions when comparing actual conditions to standard sea-level pressure.
Derivation
A millibar is one-thousandth of a bar. The word bar comes from the Greek baros, meaning weight. The unit measures the weight of the air pressing down on a given area, which is exactly what atmospheric pressure is.
Why Pilots Care
This value establishes the baseline for how an aircraft will climb, cruise, and land under standard conditions, and it is the reference used to correct altimeter readings away from sea level.
Analogy
Think of 1013.2 mb like the zero mark on a measuring scale for standard sea-level pressure. Actual pressure can be above or below it, but this number gives everyone the same reference point.
Grounding Statement
If you sealed a barometer at sea level on an average day with average weather, it would read very close to 1013.2 mb.
Intuition Check
1013.2 mb is not the pressure everywhere all the time. It is a standard reference value used for comparison.
Example Sentence 1
The altimeter setting was 1005 mb, well below the standard 1013.2 mb, so true altitude was lower than indicated.
Example Sentence 2
Before departure the pilot verified the altimeter setting matched the standard value of 1013.2 millibars.