The Collaborative International Dictionary of English v.0.48:

magnetic resonance \magnetic resonance\ n. (Physics)
   A phenomenon resulting from quantization in the magnetic
   field of electrons or atoms or molecules or nuclei exposed to
   an external magnetic field, by which a narrow frequency band
   of radio waves may be absorbed and re-emitted by the object.
   [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

magnetic resonance
    n 1: resonance of electrons or atoms or molecules or nuclei to
         radiation frequencies as a result of space quantization in
         a magnetic field