The Collaborative International Dictionary of English v.0.48:

nuclear magnetic resonance \nu"cle*ar mag*net"ic res"on*ance\ n.
   (Physics)
   The specific absorption and re-emission of electromagnetic
   radiation at characteristic wavelengths by atomic nuclei in a
   magnetic field. It is abbreviated NMR. The wavelength of
   the radiation absorbed depends on the type of nucleus, the
   intensity of the magnetic field, and the local chemical
   environment in which the nucleus resides. It is the latter
   effect (called the chemical shift), by which atoms of
   specific elements in different chemical compounds show a
   different resonance frequency, which gives rise to the
   greatest utility of this phenomenon in analyzing the chemical
   structure of substances. Similar effects of the chemical
   environment permit the discrimination of different types of
   living tissue by virtue of their different chemical
   composition, thus permitting utilization of the phenomenon in
   medical diagnostic instruments, especially for magnetic
   resonance imaging.
   [PJC]

WordNet (r) 3.0 (2006):

NMR
    n 1: resonance of protons to radiation in a magnetic field [syn:
         nuclear magnetic resonance, NMR, proton magnetic
         resonance]