What is FWHM?
Full width at half maximum (FWHM) is used when determining the spectral resolution. Here you can learn more.
Full width at half maximum (FWHM) is the width of the spectrum curve which is measured between two points on the y-axis that are half the maximum amplitude- Fig.1.
Fig.1 Full width at half maximum
FWHM is used to define resolution. If two peaks are overlapping FWHMs, it means that they are indistinguishable - they look like one peak.