Naturalism is the belief that nothing exists beyond the natural world. Instead of using supernatural or spiritual explanations, naturalism focuses on explanations that come from the laws of nature.
Beyond the belief that everything can be explained using nature, naturalism is also a term for a particular style of art and literature from the 19th century. Naturalism refers to a realistic approach to art that rejects idealized experiences. So when you look at a painting that embodies the spirit of naturalism, you'll notice it capturing the real world rather than trying to make things look better than they are. With naturalism: what you see is what you get.