What term describes any non-natural structure built on or affixed to land?

The term that best describes any non-natural structure built on or affixed to land is "Improvement." This term specifically refers to any addition or alteration that enhances the value or utility of the property, such as buildings, fences, and utilities. Improvements can significantly change how a property is used or its overall market value, thus making them a critical concept in real estate.

Other terms like "Fixture" and "Construction" refer to specific forms of improvements or the process of building, respectively, but do not encompass the broader category of all non-natural structures as effectively as "Improvement." "Enhancement," while conveying the idea of increasing value, is less formalized in real estate terminology compared to "Improvement." Thus, the most accurate term in this context is "Improvement," highlighting the significance of man-made structures in relation to land.