The Collaborative International Dictionary of English v.0.48:

Periscope \Per"i*scope\, n. [Pref. peri- + -scope.]
   A general or comprehensive view. [archaic]
   [1913 Webster]

   2. 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.
      [PJC]

WordNet (r) 3.0 (2006):

periscope
    n 1: an optical instrument that provides a view of an otherwise
         obstructed field

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "periscope":
   Aqua-Lung, air cylinder, aquascope, bathyscaphe, bathysphere,
   benthoscope, diving bell, diving boat, diving chamber,
   diving goggles, diving helmet, diving hood, diving mask,
   diving suit, scuba, snorkel, submarine, swim fins, wet suit