Plain English Meaning
It is the weight of the air pressing down at a particular place. That weight changes from day to day and from place to place, which is why pilots reset the altimeter to keep altitude readings accurate.
Full Definition
The pressure exerted by the weight of the atmosphere at a given location, measured with a barometer and typically expressed in inches of mercury (inHg) or hectopascals (hPa). It varies with altitude and weather conditions and is the value pilots set in the altimeter so it reads correct altitude.
Context Anchor
Pilots encounter barometric pressure in weather reports, altimeter settings, flight planning, and discussions of changing weather.
Derivation
From Greek baros meaning 'weight' and metron meaning 'measure.' A barometer literally measures the weight of the air, which is exactly what barometric pressure describes.