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Full Width at Half Maximum, a mathematical method for defining a peak. This method is used to generate a 'peaks' that can be used to calculate the resolution of the mass spectrometer that generated the spectrum being analysed.

The mathematical description of this process is: A full width at half maximum (FWHM) is an expression of the extent of a function, given by the difference between the two extreme values of the independent variable at which the dependent variable is equal to half of its maximum value. ( width at half maximum)