A viewfinder is the little window on a camera that you look through to see exactly what your picture will contain. You put your eye to it, move the camera until the framing looks right, and press the button. The point of it is the edges: it shows you where the picture stops. On older film cameras the viewfinder was always a small piece of glass at the top or back. On modern digital cameras many people just use the big screen instead, and some cameras have an electronic viewfinder — a tiny screen inside the eyepiece rather than plain glass. Phones have no viewfinder at all in the old sense, though people sometimes call the camera part of the phone screen 'the viewfinder' anyway, a bit loosely. It is one plain everyday word, not two, and it is countable: a viewfinder, two viewfinders. Photographers use it constantly; most other people go years without saying it.