Full Width At Half Maximum (FWHM) is a means of citing something about the width of the curved plot of a function where the plot is shaped like a hump or valley from some "usual" value to an extreme maximum or minimum. The Gaussian Function with its Bell Curve is such a function.
Astrophysics uses it as a means to quantify the width of a Spectral Line, e.g., in formulae that relate the width to other quantities. It is also used for some other functions graph such a shape such as Passband specifications.
Strömgren Photometric System