Attempting to offer a definition of Nature or even explaining what one group of thinkers meant by the term is a very difficult task -- probably harder than offering a definition of Romanticism. I am going to try to do so, even while noting that notions or concepts of Nature are likely to be varied and even contradictory. Use of the term rests on what the user believes is "natural," and there is more than a little tendency toward circular definitions of the term among even sophisticated thinkers.
Let us make our definition one "that which occurs 'naturally' or is 'naturally' found in the universe." (Yes, I know I used the word in its own definition; I warned you that we were working with a circular concept.) The problem with this definition, as applied: "Is human life part of nature or excluded?" A 20th. Century astronomer [sorry, I don't have his name handy] answered that question with a joking, "I've always imagined a couple little green whatevers circling Earth in their flying saucer, looking down at our cities and watching our satellites whiz by, and saying, 'Gee, what creatures! Isn't nature wonderful!." By his definition we, our cities, our machines and inventions, and our society are all part of nature -- a perfectly natural part of the universe. Our visitors from planet Zorg would regard us rather as we regard a beehive, a delightful manifestation of the natural world.
However, the poets we are reading tended to have a very different sense of Nature. Nature was a place where things just followed their own course, unbent by effects of culture or society. The 'natural' man [using "man" in the convention of the time, referring to both males and females] was a creature who had not been exposed to the artificial effects of culture and society. Thomas Hobbes, in the 17th. Century, had suggested that the 'natural' human life was brutish and short, and we had to have civilization to keep us from exterminating ourselves in a "war of all against all." John Locke, in the 18th. Century, suggested that we were not the evil creatures Hobbes thought we were, but were born neutral -- in fact blank -- in terms of both morals and knowledge, and that our lives were shaped for both good and evil by our environment. Locke's obvious implication is that civilization can either enlighten us or enslave us.
Many of the Romantics would have accepted the separation of Nature from culture, and would have reversed Hobbes's claim. The Romantic claim might be man is by nature good, and it is society that makes him evil. Rousseau's suggestion was such; 19th. Century American Henry David Thoreau suggested that if a man freed himself from the trappings of convention and the complications of society he might find the Truth that resides in his own heart and conscience. Wordsworth's poems (note both "Lines Composed Above Tintern Abbey" and "Ode: Intimations of Immortality") suggest that the child is an innocent, capable of seeing deeply into the richness, beauty, and ultimate purpose of Nature, and that the child loses this innocence and this ability to appreciate Nature and life through the effects of negative experience. The beauty fades, living on in memory. One steps into nature in the attempt to remember that beauty and innocence (though not to again fully experience it; by Wordsworth's conception it is gone; innocence can only be recollected, not regained).
Let's move to our definition two: the natural world. Our visitors from Zorg (above) would declare us part of the natural world, as would our 20th. Century scientist. However, most of those we are examining in this segment of the course would draw a sharp distinction between that which occurs "naturally" and that which humans create or build. With some notable exceptions, earlier generations of Europeans were shy of the natural world. In nature, the structure humans depend on to survive was absent. Instead, nature offered great storms, carnivorous beasts, hunger, and even the Devil himself. The dominant desire was to subdue nature: to "have dominion" by domesticating animals, plowing the earth, building shelters, storing food, and generally working to keep the forces of flood, disease, and famine at bay. The gardens at Versailles represent this desire for order, as the plants are arranged and pruned into close geometric patterns.
By the end of the 18th. Century, it seems, Nature began to look more inviting than man's cities. Urban squalor and unbreatheable air predated the Industrial Revolution; however, smoky mills and factories compounded urban ugliness. Plus, the human sense of freedom (Locke, etc.) was hardly enhanced by viewing rows of regimented shrubbery. That sense of freedom was enhanced by viewing Wild Nature, as frightening as it might be (Thomas Jefferson's "Natural Bridge" offers his admiration of "the sublime").
The English garden, with winding paths and irregularly spaced plants offered a "natural" alternative to the regimentation of Versailles. A trek through the English "wilderness," where generations of hunters, herdsmen, and farmers had driven off any dangerous beasts and had left winding paths through the woods and meadows, offered the opportunity to reconnect with Nature -- a gentler Nature than the wild woods of the American continent, and a gentler environment than the teeming, squalid cities. The Wordsworths and Coleridge were regular hikers, as their biographies mention.
The contrast was apparent: in the cities, the armies, and the factories one found brutish, selfish and diseased humans; in rural England one found yeoman farmers, industrious and healthy. In the cities one found foul air, smoke, human waste, and all manner of ugliness; in rural England one breathed fresh air and watched the reapers working serenely under the warm sun. That picture, I believe, influenced the Romantic conception of Nature; I believe that picture does much to explain the perspective of poets from Wordsworth (or even Gray) on through William Morris and the Victorian Romantics.