Final answer:
The window in which a JavaFX app's GUI is displayed is known as the stage, serving as the top-level container for scenes in a JavaFX application.
Step-by-step explanation:
In JavaFX, the window in which a JavaFX app's GUI is displayed is known as the stage. The stage acts as the top-level container for all JavaFX applications. It can host scenes that consist of visual elements. JavaFX applications follow a life cycle, and the stage is created by the platform itself when the application is launched. The primary job of the stage is to host the application's scenes. When you develop a JavaFX application, you start by extending the Application class and then override the start() method, which receives the primary stage as an argument.