Source : Webster's Revised Unabridged Dictionary (1913)

Diamagnetism \Di`a*mag"net*ism\, n.
   1. The science which treats of diamagnetic phenomena, and of
      the properties of diamagnetic bodies.

   2. That form or condition of magnetic action which
      characterizes diamagnetics.

Source : WordNet®

diamagnetism
     n : phenomenon exhibited by materials like copper or bismuth
         that become magnetized in a magnetic field with a
         polarity opposite to the magnetic force; unlike iron they
         are slightly repelled by a magnet