Plain English Meaning
A device that measures how much the air is pressing down at a given location.
Full Definition
An instrument that measures atmospheric pressure, typically expressed in inches of mercury ("Hg) or millibars (mb/hPa). In aviation, barometric measurements are the basis for altimeter settings, pressure altitude, and many weather reports.
Context Anchor
Seen in instrument flying, altimeter discussions, pressure altitude, and weather reports.
Derivation
From the Greek baros meaning 'weight' and metron meaning 'measure' -- literally a 'weight measurer.' That fits, because atmospheric pressure is essentially the weight of the air above you pressing down.
Intuition Check
A barometer does not measure altitude directly. It measures air pressure; altitude can be figured from pressure because air pressure generally decreases as you go higher.
Example Sentence 1
The weather station's barometer was reading 29.85 "Hg, so the pilot set that value in the altimeter's Kollsman window before takeoff.
Example Sentence 2
A steadily falling barometer indicated that weather was changing along the route.