(1) (upper case "W") Refers to Microsoft Windows: computers running Windows and/or applications running under Windows. See
Windows and
Windows versions.
(2) (lower case "w") Also called a "windowing environment," it refers to any operating system that supports multiple on-screen windows via a window manager; for example, Windows, Mac, Unix and Linux. The term may also refer to an application that provides multiple windows for documents or pictures. See
window manager.