The Collaborative International Dictionary of English v.0.48:

Peephole \Peep"hole`\, n.
   A hole, or crevice, through which one may peep without being
   discovered.
   [1913 Webster]

WordNet (r) 3.0 (2006):

peephole
    n 1: a hole (in a door or an oven etc) through which you can
         peep [syn: peephole, spyhole, eyehole]

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "peephole":
   Texas tower, air hole, armhole, beacon, belvedere, bleachers,
   blowhole, bridge, bullet-hole, bunghole, conning tower, cringle,
   deadeye, eye, eyelet, gallery, gasket, gazebo, grandstand, grommet,
   guide, keyhole, knothole, lighthouse, lookout, loop, loophole,
   manhole, mousehole, observation post, observatory, outlook,
   overlook, peanut gallery, pharos, pigeonhole, pinhole, placket,
   placket hole, porthole, punch-hole, ringside, ringside seat,
   sighthole, spiracle, tap, top gallery, tower, vent, venthole,
   watchtower