Formal:
* Scuttle: This is a more formal term often used in nautical contexts.
* Portlight: This term is also used in nautical contexts and is more specific to a window in a ship's hull.
Informal:
* Window: This is a general term that can be used to describe a porthole.
* Round window: This emphasizes the circular shape of a porthole.
* Viewpoint: This is a more metaphorical synonym, emphasizing the porthole's function as a source of perspective.
The best synonym for "porthole" depends on the specific context and tone of your writing.