What is the name of Microsoft's first graphical operating system?

Windows 1.0 was Microsoft's first graphical operating system, or more precisely, its first major graphical Windows operating environment. It was released in the United States on November 20, 1985, and ran on top of MS-DOS.

Microsoft publicly demonstrated the project in 1983 after Bill Gates had seen graphical-computing concepts such as VisiCorp's Visi On. Windows 1.0 added mouse support, tiled application windows, menus, and a graphical environment for programs that otherwise would have been run from the command line. It included early versions of Paint, Notepad, Calculator, Write, Clock, and Terminal.

The phrase “operating system” is sometimes used loosely here: Windows 1.0 was technically a graphical shell and operating environment extending MS-DOS, rather than a fully independent operating system in the modern sense. It also looked very different from later Windows because its windows could not overlap. Windows 2.0 succeeded it in 1987, while Windows 3.0 brought much greater popularity in 1990.