Two Imperial officers are standing before a window on the Executor.
A viewport, which can also be called a window or porthole, was essentially a frame holding a transparent material, constructed into a wall to allow light to enter. It was typical for starships and space stations, as well as buildings on planets, to have these.
If a viewport was built into a roof, it was known as a skylight.
The main material used for both viewscreens and windows on starships was transparisteel, an alloy made from the mineral lommite. This mineral was the primary export of the planet Eriadu. Glass was another material that saw use.