(covers information from several alternate timelines)
A viewport, or viewing port or simply window, is a transparent medium used to see through a wall so as to view the outside of a starship or building.
While on board the replica USS Enterprise, James T. Kirk, who believed he was on the real USS Enterprise, lead Odona to a viewing port, explaining to her that through it, they will be able to see outside. Upon peering through it, Kirk saw a mass of Gideons, rather than space. (TOS: "The Mark of Gideon")
Data once told Admiral Alidar Jarok, that the middle viewport in Ten Forward aboard the USS Enterprise-D was considered by many crewmembers to be their favorite on the whole ship because of the breathtaking view. (TNG: "The Defector")
Window would probably be the word Doctor Julian Bashir would say instead of tricorder to name this technical device when he would've suffered from the aphasia virus, as he told Commander Sisko. (DS9: "Babel")
Major Kira Nerys told Mullibok a story from her childhood while treating him on Jerrado. The story was about a huge ugly tree in front of her window. It was so huge and ugly even the birds won't sit on it and he derailed the sun from falling into the window. (DS9: "Progress")
When Derran Tal caught Harry Kim staring out the window of her quarters on board the Varro generational ship, she told him "if you're planning on leaving through the viewport, you might want to get dressed. The interstellar vacuum might get a little chilly." (VOY: "The Disease")
Some windows featured colored glass for artistic purposes. In 1986, Feinberg's Loan and Pawn had a stained glass window out front featuring a bowl of fruit. (Star Trek IV: The Voyage Home) Stained glass windows were often found in religious settings. St. Mary's Church had stained glass windows. (VOY: "Spirit Folk")