Source : Webster's Revised Unabridged Dictionary (1913)

Plethysmograph \Pleth"ys*mo*graph\, n. [Gr. ? an enlargement +
   -graph.] (Physiol.)
   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. --
   {Pleth`ys*mo*graph"ic}, a.

Source : WordNet®

plethysmograph
     n : 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)