When refering to a camera, this means Single Lens Reflex. These are the more fancy cameras where often the lens can taken off the body of the camera and exchanged for another lens. The basic thing is that the use a mirror inside the camera which either reflects the image caught by the lens to the viewer of the camera, or to the film in the camera when shooting a picture. This way the end result will be a picture matching exactly what you saw in the viewer/finder of the camera.
Acronym for Single Lens Reflex, a type of camera that enables the photographer to see the scene through the lens that takes the picture. A reflex mirror reflects the scene through the viewfinder. The mirror retracts when the shutter-release button is pressed.
Single Lens Reflex camera. One lens acts as both viewing and taking lens.