An array of the frequency components of the MR signal according to frequency. Nuclei with different resonant frequencies will show up as values at different corresponding frequencies in the spectrum. When resonances are relatively isolated they appear as peaks or lines in the spectrum.
A spectrum is also a graphic representation of the range over which a quantity extends.

Continuum of electromagnetic energy.

Convolution differentiation is a method of suppressing broad underlying spectral lines in order to emphasize narrower spectral lines. Strong smoothing of the spectrum (e.g., by severe negative exponential weighting of the time data) will suppress the narrow lines but minimally affect very broad ones; subtracting such a smoothed spectrum from the original will largely remove the contributions from the broad lines. This provides a means of baseline correction.

Multiplication of the time-dependent signal data by an exponential function, exp(t/TC), where t is time and TC is a parameter called the time constant (in spectroscopy). The time constant can be chosen to either improve the signal to noise ratio (with a negative TC) or decrease the effective spectral line width (with a positive TC) in the resulting spectrum. The use of a negative TC to improve SNR is equivalent to line broadening by convolving the spectrum with a Lorentzian function of corresponding reciprocal width.

Nuclei can retain their magnetic orientation through a chemical reaction. Thus, if RF radiation is supplied to the spins at a frequency corresponding to the chemical shift of the nuclei in one chemical state so as to produce saturation or inversion, and chemical reactions transform the nuclei into another chemical state with a different chemical shift in a time short compared to the relaxation time, the NMR spectrum may show the effects of the saturation or inversion on the corresponding, unirradiated line in the spectrum. This technique can be used to study reaction kinetics of suitable molecules.