The full width at half maximum (FWHM) is the width of a line shape at half of its maximum amplitude, as shown below:
A closely related quantity is the half width at half maximum (HWHM) or the Resolving Resolution and it is half of the FWHM. For Gaussian line shapes, the FWHM is about 2.4 standard deviations. While the concept is simple, this is a vital quantity; the FWHM is used to define resolution. If two peaks have overlapping FWHMs, they are unresolvable, i.e. they will look like one peak.
