full width at half maximum (FWHM)
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.
Note 1: FWHM is applied to such phenomena as the duration of pulse waveforms and the spectral width of sources used for optical communications.
Note 2: The term full duration at half maximum (FDHM) is preferred when the independent variable is time.