What is a manometer?

Origin: manomètre: coined in 1705 by Pierre Varignon a French mathematician ... from the Greek manos, rare (taken in sense “thin, sparse”) + French -mètre, -meter

A double-leg liquid-column gage used to measure the difference between two fluid pressures. Manometers are precision instruments which typically consist of a tube marked with a scale and containing a relatively incompressible fluid, such as mercury. The barometer is a special case of manometer with one side of the tube closed creating a pressure of zero absolute.

I think anyone involved in pressure measurement and calibration probably learned about manometers from their high school science teacher, but "manometer" has morphed into a term used to describe any precision pressure measuring device.

Back in the day, one of Mensor's first products was the Quartz Manometer. It used a quartz bourdon tube as the pressure sensor; no double-leg tube and no mercury involved at all.