Android is a software stack but not an OS. as stated by developer.android.com
What is Android? Android is a software stack for mobile devices that includes an operating system, middleware & key applications. The Android SDK provides the tools and APIs necessary to begin developing applications on the Android platform using the Java programming language.
I don't understand that software stack is OS works with the Applications, so Windows is an OS but if I install Applications on it, should I call it a software stack?
So what is the difference between Software Stack and OS, Android includes kernel so why not its an OS?