Toplevel

Toplevel, often written as toplevel or top-level, refers to the outermost scope or container in a system. In software, it commonly describes code or definitions that exist outside any function, class, or block; in user interfaces, it can refer to a top-level window that exists independently of other windows.

In programming languages, toplevel declarations are those not nested inside functions or methods. They define a module’s or program’s public surface: module-level constants, functions, and classes. Some languages treat top-level forms as code executed when a file is loaded, compiled, or interpreted. They contrast with nested scopes; in languages with modules and namespaces, the toplevel scope is the module or file itself. In Lisp-family languages, toplevel forms are evaluated at load time or in the top-level REPL.

In GUI toolkits, a toplevel window is a top-level window managed by the window manager, not a child of another window. These windows have independent title bars and decorations and can host various widgets. Additional toplevel windows can be created to display auxiliary dialogs without embedding them in the main window.

The term’s precise meaning depends on context, but it generally denotes outer, independent, or root-level elements in a system, as opposed to nested or contained elements.