Plethysmograph

noun

1. An instrument that measures variations in the size of an organ or body part on the basis of the amount of blood passing through or present in the part.
2. An instrument for obtaining tracings indicating the changes in the volume of a part of the body, especially as dependent on the circulation of blood in it.
3. An instrument for determining and registering the variations in the size or volume of a limb, as the arm or leg, and hence the variations in the amount of blood in the limb.
4. An instrument for measuring changes in volume within an organ or whole body (usually via fluctuations in the amount of fluid it contains).
5. a measuring instrument for measuring changes in volume of a part or organ or whole body (usually resulting from fluctuations in the amount of blood it contains)