Single-lens Reflex (SLR)
A camera design in which a hinged mirror reflects the taking lens's image up into the viewfinder, then swings clear when the shutter fires.
In depth
The single-lens principle solved the parallax problem that plagues every viewfinder camera: what you see through the finder is exactly what the lens sees, at whatever focal length is fitted. The cost is a mirror box that must be long enough to swing, which forces wide-angle lenses into complex retrofocus designs, and a brief blackout at the moment of exposure.
Examples in practice
Fitting a 400 mm lens to an SLR frames the shot precisely, where a rangefinder would need an accessory finder and guesswork.