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A multi sided glass prism housed in the roof of an single-lens reflex camera so that the image that's seen be the lens can be viewed through an optical viewfinder above the lens. The light enters through the lens is reflected up by a reflex mirror to the prism and across to the eye.
An SLR or single-lens reflex camera is really designed for the enthusiast or professional photographer, or for the person who can put up with a larger camera in return for increased accuracy and greater versatility. This type of camera has through the lens viewing with a mirror behind the lens and a pentaprism to direct the light passing through the lens to the optical finder. The mirror lifts up out of the way as a photograph is taken. As you look through the lens that takes the picture, the composition can be more accurate. And in most cases you can exchange the lenses, giving you a wider scope of options. The metering and focusing systems are usually more accurate too. Despite all this creativity it's still possible to put most SLRs in a full auto point & shoot mode so anyone could use one with ease, but don't expect to fit one in a pocket! They are much bigger than that. A modern, digital version of the SLR is called a DSLR.
As you look through the viewfinder of an SLR it should give you as near as possible what will appear in your image. Because of the design, position and size of mirror/pentaprism, etc., this is not always possible and you often see less than 100%. The higher this figure is, the more accurate the results will be. It's usually only professional cameras that give 100% accuracy.