periscope

n. optical scope that allows one to see objects that are located above one's line of sight

A periscope is an instrument for observation from a concealed position. In its simplest form it is a tube in each end of which are mirrors set parallel to each other and at an angle of 45° with a line between them. A periscope may be used as a toy or for seeing over people's heads in a crowd. This form of periscope, with the addition of two simple lenses, was used for observation purposes in the trenches during World War I. Periscopes are also used in some gun turrets and armored vehicles.More complex periscopes, using prisms instead of mirrors, and providing magnification, are used on submarines. The overall design of the classical submarine periscope is very simple: two telescopes pointed into each other. If the two telescopes have different individual magnification, the difference between them causes an overall magnification or reduction.
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Noun
1. an optical instrument that provides a view of an otherwise obstructed field
(hypernym) optical instrument
(part-holonym) submarine, pigboat, sub, U-boat

n. periscope, optical scope that allows one to see objects that are located above one's line of sight

(n.)
an optical instrument of tubular shape containing an arrangement of lenses and mirrors (or prisms), allowing a person to observe a field of view otherwise obstructed, as beyond an obstructing object or (as in submarines) above the surface of the water.
  

(n.)
A general or comprehensive view.