Nature refers to the natural environment. It can also refer to the physical world as a whole.
Religion and Nature
In almost all of the world's religions, there are beings who are said to represent or control natural events. For example, in Egyptian religion, Geb was the god of the earth. It was believed that earthquakes were caused by Geb's laughter.
Naturalism is the belief that the universe and the systems within do not need a supernatural force or explanation.  By definition, it is much more comprehensive than atheism, since it applies to the rejection of all supernatural ideas (i.e. afterlife, ESP, the soul, prayer and some forms of alternative medicine) as opposed to simply a disbelief in a god.