The Collaborative International Dictionary of English v.0.48:

Pleximeter \Plex*im"e*ter\, n. [Gr. ? stroke, percussion (from ?
   to strike) + -meter.] (Med.)
   A small, hard, elastic plate, as of ivory, bone, or rubber,
   placed in contact with body to receive the blow, in
   examination by mediate percussion. [Written also
   plexometer.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

pleximeter
    n 1: a small thin metal plate held against the body and struck
         with a plexor in percussive examinations [syn:
         pleximeter, plessimeter]