RhymeZone: brightener definitions

Definitions of brightener:

noun: One who or that which brightens.
noun: A chemical agent that absorbs ultraviolet and violet light and re-emits blue light by fluorescence, used to make fabric or paper look whiter.
noun: (metallurgy) Any substance added to an electrolytic bath to yield a brighter electroplated surface.

(Definitions from Wiktionary)