Source : Webster's Revised Unabridged Dictionary (1913)

Porthole \Port"hole`\, n. (Naut.)
   An embrasure in a ship's side. See 3d {Port}.

Source : WordNet®

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}]