Plain English Meaning
A small unit used to measure how much the air is pressing down. The higher the number, the higher the pressure.
Full Definition
A unit of atmospheric pressure equal to one one-thousandth of a bar, or 100 pascals. Standard sea-level atmospheric pressure is 1013.2 millibars, equivalent to 29.92 inches of mercury.
Context Anchor
Seen in weather reports, weather charts, altimeter-setting discussions outside the United States, and some aircraft or engine performance data.
Derivation
From Latin 'mille' meaning 'thousand' and 'bar' from Greek 'baros' meaning 'weight.' A millibar is one-thousandth of a bar, where a bar represents roughly the weight of the at . Knowing this makes the unit easier to picture: it is a small slice of the total weight of the air above you.