A viewfinder system used in twin lens reflex cameras, many other medium format cameras, and some 35mm cameras. As implied by the name, with a waist level viewfinder, you hold the camera at waist level and look down at it to see the image. The image is reversed left-right because of the way the mirror reflects the image. By holding the camera at waist level, the photographer is not so obvious, so it is possible to take pictures that are more candid.
The other advantages of the waist level finder are cost and weight. Most medium format cameras ship with a waist level finder, and a prism finder, which is generally standard in 35mm cameras is available for several hunderd dollars more. With medium format cameras, forgoing the prism and using a waist level finder reduces the weight of a camera by a pound or two.