What is the term for an opening in the facade of a building?

The term "bay" refers to a significant architectural feature in the facade of a building. In architectural terminology, a bay is typically defined as a space that is marked by structural divisions and often includes openings, such as windows or doors, allowing for light and air to enter while also contributing to the aesthetic harmony of the facade. In many cases, a building's bay can also provide a rhythm to the facade, breaking up the mass and allowing for visual interest.

While a "window" is indeed an opening in the wall designed to allow light and visibility, it is a more specific feature and does not encompass the broader concept that a bay represents. An "arch" refers to a curved structure that spans an opening, rather than the opening itself. A "portal," while it can refer to a grand entrance, is more commonly associated with larger, monumental entries or gateways, which may not be applicable to all facades. Thus, "bay" correctly identifies the concept of an opening within the context of architectural proportions and is the most fitting choice among the options provided.