There's a lot more to an operating system than how it manages non-fullscreen applications. "Windows" is obviously a related term, but I don't think it's generic--when I boot up Linux on my laptop, I don't say, "This Linux/Xorg/Openbox combination sure is a great windows!" But I might say, "This collection of software for my smartphone compiled and sold by Amazon sure is a great app store."

Edit: See also: MS Office, MS Word, MS Security Essentials, and arguably MS Internet Explorer and Windows Explorer (I browse the internet and manage/browse files--there's not much literal "exploring" going on)