What is "reflex camera"

n. A camera that permits the photographer to view the exact image that will be captured, unlike earlier viewfinder cameras.

n. camera that allows the photographer to view and focus the exact scene being photographed

A reflex camera is a camera that permits the photographer to view the image that will be seen through the lens, and therefore to see exactly what will be captured, contrary to viewfinder cameras where the image could be significantly different from what will be captured. A single-lens reflex camera typically uses a mirror and prism system (hence "reflex", from the mirror's reflection) to accomplish this. The mirrorless interchangeable-lens camera achieves the same result by providing the photographer with a digitally captured image. The twin-lens reflex camera provides both a viewfinder image and the reflected image from the actual camera lens.