Graphical user interface builder

A graphical user interface builder (or GUI builder), also known as GUI designer or sometimes rapid application development (RAD) integrated development environment (IDE), is a software development tool that simplifies creating graphical user interfaces (GUIs) by allowing a designer to arrange graphical control elements (often called widgets) using a drag-and-drop WYSIWYG editor. Without a GUI builder, a GUI must be built by manually specifying each widget's parameters in the source code, with no visual feedback until the program is run.

Source: Wikipedia — Graphical user interface builder (CC BY-SA 4.0)