nature is the physical world and its forces considered apart from human works — and, in a sense at least as old and still fully live, the essential character or constitution of a thing or person.
Both senses are ordinary English and context does the sorting. The character sense appears in the nature of the problem, human nature, good-natured, and second nature, where the word names what something fundamentally is rather than where it is found. The environmental sense — nature as the material world beyond human civilization — dates only from the 1660s, and its arrival built a division into the word: it places human activity outside nature, a framing later ecological and philosophical writers have contested at length. Earlier English usage had run in the other direction, treating nature as the forces or processes of the material world including everything in it, a sense recorded from the mid-fourteenth century.
The two senses meet in the phrase nature and nurture, current since Shakespeare's era and given its standard definition by Francis Galton: "Nature is all that a man brings with himself into the world; nurture is every influence from without that affects him after his birth." Here nature means inherited constitution, the character sense applied to a person, rather than the outdoors.
The word came into English in the late thirteenth century from Old French nature, from Latin natura, "birth, constitution, character, the course of things," built on natus, the past participle of nasci, "to be born," from the Proto-Indo-European root gene-, "give birth, beget." That root ties nature to native, nation, innate, genesis, and gene — a lineage in which nature is first of all what a thing is born as, and only later the world it is born into. The personification Mother Nature is attested from the late fourteenth century.