NMR = Nuclear Magnetic Resonance is the resonance that occurs when a nucleus
(usually hydrogen, but any nucleus that has non-zero spin will work) is placed in a
magnetic field and is 'swept' by a radio frequency that causes the nuclei to
'flip'.
This causes the radio frequency to be absorbed, which is what is measured.
MRI = Magnetic Resonance Imaging is a more complex application of NMR in which the
geometric source of the resonances are detected and deconvoluted by Fourier
transform analysis.
ESR = Electron Spin Resonance is also a resonance phenomenon, except in this case
it is the spin of an unpaired electron that is in resonance, rather than a nuclear
spin.
Answered by: Vince Calder, Ph.D., Physical Chemist, retired
'On the mountains of truth you can never climb in vain: either you will reach a point higher up today, or you will be training your powers so that you will be able to climb higher tomorrow.'