The nuclear Overhauser enhancement or nuclear Overhauser effect is the fractional change in intensity of one NMR line when another resonance is irradiated in a double irradiation experiment. Nuclear Overhauser Effects are due to dipolar interactions (through space) between different nuclei and are correlated with the inverse sixth power of the internuclear distance. Nuclear Overhauser effects are customarily quoted in percent of the unperturbed resonance intensity.
From: Wüthrich, K. (1986). NMR of Proteins and Nucleic Acids, John Wiley & Sons, New York.