The Collaborative International Dictionary of English v.0.48:

constantan \constantan\ n.
   a copper-nickel alloy composed of approximately 55 per cent
   copper and 45 per cent nickel, with high electrical
   resistance and a low temperature coefficient; it is used as
   resistance wire and in thermocouples.

   Note: It has a resistance factor [rho] of 44.1 x 10^-6
         ohm-cm at 0[deg] C, in comparison to the value 1.71 x
         10^-6 ohm-cm for pure copper and 7.8 x 10^-6 ohm-cm
         for pure nickel. It is similar to the alloy Eureka,
         which has a [rho] = 47 x 10^-6 ohm-cm. --Handbook of
         Ch. 61.
         [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

constantan
    n 1: an alloy of copper and nickel with high electrical
         resistance and a low temperature coefficient; used as
         resistance wire [syn: constantan, Eureka]