As many others have already stated, a universal definition for the concept of “nature” doesn’t exist, and perhaps one doesn’t need to exist. After all, its ambiguity sparked this discussion! I don’t know about you, but I find discussing ambiguous or controversial concepts insightful and eye-opening. Moreover, because every human differs slightly to radically from other humans, no single concept or definition will ever be agreed upon by the whole of humanity.
Philosophical ramblings aside (or not), I define nature as quite literally everything, the entire Universe and its inherent processes. Living organisms are just as natural as minerals or stars. It simply turns out that the constituent particles of a human are arranged so differently than the Sun’s that the former bears the burden of consciousness and the latter will never perceive its own existence.
From a pragmatic standpoint, I am tempted not to define nature as quite literally everything, but it’s easier than constructing an exclusionary definition that says “this is natural, this is not”.