The phrase "round window" refers to a circular opening in a surface such as a building, vehicle, or machinery. Synonyms for the word "round window" include porthole, peephole, eyehole, aperture, orifice, and opening. These words all describe an opening that allows light or air to enter or exit a confined space. Additionally, depending on the context, a round window could also be called a vent, portal, hatch, or access point. The use of these synonyms can help to provide variety in language and add color and depth to writing or speech.