An operating system provides the lowest stable level of a userspace API to the underlying hardware capabilities. This might be a bare kernel, or a kernel plus b

An operating system provides the lowest stable level of a userspace API to the underlying hardware capabilities. This might be a bare kernel, or a kernel plus bundled runtime libraries. Debian is a versioned and tested copy of GNU/Linux, which is an operating system consisting of the Linux kernel and the GNU libc. Linux can be also be an OS on its own, because its kernel<->userspace API is stable. This is not true of all kernels, for example macOS does not guarantee stability below libSystem. Some people extend "operating system" up into userland tools that are bundled by the vendor or specified by a standard like POSIX, for example `ls` or `initd`. I think this is stretching the meaning a bit, because to the kernel there's no difference between `ls` and (for example) Firefox.