IPA: /ˈdɑːrkruːm/
KK: /ˈdɑrkˌrum/
A room used for developing photographs, where light is controlled to protect sensitive materials.
The photographer spent hours in the darkroom developing his latest photos.
Darkroom → The word is formed from "dark" (from Old English *deorc*, meaning lacking light) and "room" (from Old English *rum*, meaning space or area). A darkroom is a space that lacks light, typically used for developing photographs.
Think of a space that is 'lacking light' ('dark') and is a 'space' ('room') — that's what a darkroom is used for.