Plain English Meaning
It is the unit most of the world uses to measure air pressure for altimeter settings, instead of the inches of mercury used in the United States.
Full Definition
A unit of atmospheric pressure used on aviation altimeters and weather reports outside the United States. One hectopascal equals 100 pascals, and 1 hPa is numerically identical to 1 millibar. Standard sea-level pressure is 1013.25 hPa, which corresponds to 29.92 inches of mercury (inHg) used in the U.S.
Context Anchor
Seen in international operations, foreign weather reports, approach charts, and altimeter setting instructions.
Derivation
From 'hecto-' (Greek hekaton, meaning one hundred) and '' (named after Blaise Pascal, the 17th-century French scientist who studied pressure). So a hectopascal is literally 100 pascals — a convenient for everyday atmospheric pressure values.