The Collaborative International Dictionary of English v.0.48:

Thermopile \Ther"mo*pile\, n. [Thermo- + pile a heap.] (Physics)
   An instrument of extreme sensibility, used to determine
   slight differences and degrees of heat. It is composed of
   alternate bars of antimony and bismuth, or any two metals
   having different capacities for the conduction of heat,
   connected with an astatic galvanometer, which is very
   sensibly affected by the electric current induced in the
   system of bars when exposed even to the feeblest degrees of
   heat.
   [1913 Webster]

WordNet (r) 3.0 (2006):

thermopile
    n 1: a kind of thermometer for measuring heat radiation;
         consists of several thermocouple junctions in series