A .nib file, short for "NeXT Interface Builder" file, is a resource file primarily used in Apple's macOS and iOS application development. Originating from the NeXTSTEP operating system, it stores the graphical user interface (GUI) elements and their layout for an application. These files are created and edited using Interface Builder, a visual design tool integrated into Apple's Xcode development environment. A .nib file contains serialized objects that represent windows, views, buttons, text fields, and other UI controls, along with their properties, connections (outlets and actions), and layout constraints. When an application launches, the runtime loads the .nib file, instantiating these objects and constructing the user interface as defined. While .nib files were historically the primary format, they have largely been superseded by .xib (XML Interface Builder) files and, more recently, .storyboard files, which offer more comprehensive UI design capabilities, especially for multi-screen applications and navigation flows. Despite the evolution, the underlying concept and runtime loading mechanism remain similar. .nib files are crucial for separating UI design from application logic, facilitating collaborative development and easier UI modifications.