The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster] -- Pleth`ys*mo*graph"ic, a.
   [1913 Webster]

WordNet (r) 3.0 (2006):

plethysmograph
    n 1: 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)