Windows, the operating system released in November 1985, takes its name from windowing systems, the term for Graphical User Interfaces (GUIs) at the time.
It was initially going to be named 'Interface Manager' before Microsoft's head of marketing Rowland Hanson convinced the company that they needed a more user-friendly name.
"With regard to Windows, we were actually fortunate to be late with our GUI (Graphical User Interface) introduction, "Hanson said. "Other GUIs had been launched ahead of us so we were able to review how the opinion leading editors were describing them. The common description of the GUIs at that point of time was that they were “windowing systems” for your desktop. Therefore, if all these GUIs already launched ahead of the Microsoft GUI were being described as “windowing systems”, the name “Windows” became essential; not optional."
Hanson adds that they essentially realised that rather just being another "windowing system" by choosing the name Windows they "defined the category and everybody else reinforced our selected name".
Sources: Windows is named Windows: But why?.