IPA: /ˈdɑrkənər/
KK: /ˈdɑrkənər/
An object or substance that makes something darker or reduces the amount of light.
The artist used a darkener to create shadows in the painting.
Darkener → It is formed from "dark" (from Old English *deorc*, meaning lacking light) and "-er" (meaning a person who). The word "darkener" refers to a person or thing that makes something darker or less illuminated.
Think of someone or something that makes things 'lacking light' ('dark') — that's why a darkener is a person or thing that darkens.