Instruments used to measure pressure are called pressure gauges or vacuum gauges. Pressure is defined as force per unit area that a fluid exerts on its surroundings. You can measure this force by detecting the amount of deflection on a diaphragm positioned inline with the fluid. Given the known area of the diaphragm, pressure can then be calculated. Pressure sensors are packaged with a scale that provides a method to convert to engineering units.
The SI unit for pressure is the Pascal (N/m2), but other common units of pressure include psi, atmospheres, bars, inches of mercury, millimeters of mercury, and torr.