Full-Width Half Maximum

Also known as FWHM. A measure of the size of a star or other image. Half maximum means the distance from the center at which the brightness drops to one half of the center value. Full width means the measurement is given as a diameter rather then a radius. The FWHM of a point-source in an image, divided by two to get a radius, can be used as an estimate for the radius of the point-spread function model.