Half the width of a spectrum line (or in some cases the full width) measured at half its height.
Astronomy
Seefull width at half maximum.
Chemistry
Half the width of a spectrum line (or in some cases the full width) measured at half its height.
Geology and Earth Sciences
Half the width of an anomaly measured between points with half the peak amplitude. It is used when determining depths to the centres of causative bodies in gravity and magnetic profiles.