“Linux” has two meanings. One of them is the kernel itself; another is a collection of operating systems, that Stallman would call “GNU plus Linux” instead.

The later involves two factors. A “hard” one is the presence of the Linux kernel; but there’s also two softer and fuzzier ones:

• the operating system behaves like other OSes that the user calls “Linux”. For example you’re expected to have a /home/username, you can install a different DE/WM if you want, this kind of stuff.
• the OS is open in letter and in spirit. This is ideological but ideology is damn important when dealing with Linux.

A good example of both is ChromeOS: people don’t usually call it “Linux”, even if it uses the Linux kernel. It’s simply too atypical in behaviour, and ideologically too distant from the open source movement.