According to Merriam-Webster’s medical dictionary, Spirometry is the measurement by means of a spirometer of the air entering and leaving the lungs. The same dictionary defines a spirometer as an instrument for measuring the air entering and leaving the lungs. There is circularity in these definitions, unfortunately. A better definition would be simpler: spirometry is the measurement of the air entering and leaving the lungs. A spirometer, then, is any of a number of devices used today or historically to accomplish spirometry.
Spirometry
