a physical boundary is a naturally occurring divide between two areas. Unlike things like politically-drawn state lines, boundaries are not man made. Instead, they are part of nature. A great example of a physical boundary is a mountain. For example, the Pyrenees Mountains, which serve to separate France and Spain, are a physical boundary; so are the Ural Mountains which separate Europe and Asia.

Along with mountains, oceans, seas, even deserts are examples of physical boundaries. For example, Africa is often referred to in terms of Saharan and sub-Saharan Africa, as in part of the desert or south of the desert.