Merriam-Webster Collegiate Dictionary - plethysmograph

 noun  Etymology: International Scientific Vocabulary, from Greek plethysmos increase, from plethynein to increase, from plethys mass, quantity, from plethein  Date: 1872 an instrument for determining and registering variations in the size of an organ, limb, or part resulting from changes in the amount of blood present or passing through it  • ~ic adjective  • ~ically adverb  • ~y noun