Parallax-

The apparent displacement of an observed object due to a change in the position of the observer.

Pentaprism-

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.

Periscope-

Any of various tubular optical instruments that contain reflecting elements, such as mirrors and prisms, to permit observation from a position displaced from a direct line of sight.

View Finder-

A viewfinder is what the photographer looks through to compose, and in many cases to focus on the picture.