Definition
The international standard sea level atmospheric pressure value, equal to 29.92 inches of mercury or 1013.2 millibars. It is the pressure setting pilots place in the altimeter's Kollsman window when flying at or above the transition altitude in regions that use hectopascals, so that all aircraft display pressure altitude (flight levels) on a common reference.
Plain English
It is the standard air pressure used worldwide as a baseline. When set in the altimeter, every aircraft is measuring height from the same imaginary reference, so flight levels match between aircraft.
Context Anchor
Seen in altitude, altimeter setting, standard atmosphere, and flight level discussions, especially in instrument flying.
Derivation
Hectopascal comes from 'hecto-' (Greek hekaton, meaning one hundred) and 'pascal,' the SI unit of pressure named after Blaise Pascal. One hectopascal equals 100 pascals, and it happens to be numerically identical to one millibar — which is why 1013.2 hPa and 1013.2 mb are the same value.
Why Pilots Care
Altimeter settings are referenced to this value; using it instead of local pressure produces pressure altitude, which is required for flight levels and performance calculations.
Grounding Statement
Think of 1013.2 hectopascals as the agreed pressure baseline pilots use when everyone needs to measure altitude from the same pressure reference.
Intuition Check
Do not read 1013.2 hectopascals as the actual pressure outside the airplane. It is a standard reference value unless the current air pressure happens to match it.
Example Sentence 1
Climbing through the transition altitude, the pilot set 1013.2 hectopascals in the altimeter and reported reaching FL180.
Example Sentence 2
Because local pressure was lower than 1013.2 hectopascals, the aircraft was actually lower than the altimeter indicated.