The story
From single + lens (contrasted with a twin-lens reflex camera, TLR), and reflex (“reflection”), the principle by which the image path is diverted.
Etymology from Wiktionary, CC BY-SA 4.0.
single-lens reflex
Today it means: Denoting or relating to a type of camera which allows the photographer to view the subject through the same lens that will be used to take a photograph, usually by incorporating a moving mirror and prism. Full definition →