The manometer was invented in the 16th century by the Italian scientist Evangelista Torricelli. He carried a manometer up a mountain and found that the air pressure decreases as altitude increases. He also noted that the air pressure decreases just before a storm and is at its highest on a nice day. The air manometer can be used as a barometer.
Manometers can give pressures in terms of a column of mercury (like the physician's blood pressure sphygmomanometer) or in terms of a column of air (most air manometers). In some circles, water is used--divers have a rule of thumb that 10 feet of water exerts about the same pressure as one atmosphere.
An air manometer is any device that measures pressure and reports the measurement in terms of a column of air. Sometimes this is reported in "atmospheres"--the pressure that the atmosphere exerts at sea level on a normal day. This is about 15 pounds per square inch, or 760 Torr.