What is the meaning of full width half maximum (FWHM)?

Full width at half maximum (FWHM) is used to designate the width of any intensity distribution at half the maximum intensity. In particular, the width at half maximum is used to designate the width of spectral lines or as a measure of the narrow bandwidth of monochromatic light.
The term bandwidth is also often used instead of FWHM.

Monochromator
Where is full width at half maximum (FWHM) used?

The width of spectral lines is often given by FWHM.

 

What is the difference between full width half maximum (FWHM) and bandwidth?

These terms are synonyms.