FXML stands for FXML (or FX Markup Language) and is an XML-based markup language used primarily for defining the user interface (UI) of applications developed using JavaFX, a modern GUI toolkit for Java. It separates the presentation logic (the look and layout of the UI components like buttons, text fields, and panes) from the application's business logic, which is typically written in Java. This separation of concerns makes applications easier to design, maintain, and evolve. FXML files describe the scene graph—the structure and properties of the UI elements—in a declarative manner, similar to how HTML describes web pages. Developers can define complex layouts, set properties, link event handlers, and even define styling using CSS within the FXML structure. While JavaFX is the primary context, the underlying structure is standard XML, meaning it can be parsed by any standard XML parser. It is crucial for building rich, cross-platform desktop applications using the Java ecosystem.