The Collaborative International Dictionary of English v.0.48:

Porthole \Port"hole`\, n. (Naut.)
   An embrasure in a ship's side. See 3d Port.
   [1913 Webster]

WordNet (r) 3.0 (2006):

porthole
    n 1: a window in a ship or airplane
    2: an opening (in a wall or ship or armored vehicle) for firing
       through [syn: port, embrasure, porthole]

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "porthole":
   air hole, armhole, bay, bay window, blowhole, bow window,
   bullet-hole, bunghole, casement, casement window, cringle, deadeye,
   eye, eyelet, fan window, fanlight, gasket, grille, grommet, guide,
   keyhole, knothole, lancet window, lantern, lattice, light, loop,
   loophole, louver window, manhole, mousehole, oriel, pane, peephole,
   picture window, pigeonhole, pinhole, placket, placket hole, port,
   punch-hole, rose window, skylight, spiracle, tap, transom, vent,
   venthole, wicket, window, window bay, window glass, windowpane