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1660s, from Greek baros "weight" (from barys "heavy;" see grave (adj.)) + -meter. Probably coined (and certainly popularized) by English scientist Robert Boyle (1627-1691).
An instrument that measures atmospheric pressure.
Note: In general, when the barometer falls in response to a drop in pressure, bad weather is approaching; when the barometer rises because of an increase in pressure, good weather will follow.