Single Lens Reflex: A camera with a mirror that reflects the image from the lens into the viewfinder and moves out of the way to take the picture (this is the "click" sound you hear and now is added to digital cameras to make it sound more real).

SLR cameras often support interchangeable lenses, so if someone asks if you have an SLR camera they frequently will be asking about the removable lens feature and not the optical viewfinder.