Windows

Windows is a generic name for a series of loosely-related operating systems developed by Microsoft. (The earliest versions, through Windows 3.2, were 16-bit shells that ran on top of DOS rather than true operating systems, while the 9x line (Windows 95, 98, and Me) were 32-bit shells on top of DOS.)

Not to be confused with the windows that are the basis of nearly all modern graphical user interfaces (GUIs). See the Wikipedia article for a thorough discussion of what a window is.