What term is used for any non-natural structure built on or affixed to land?

The term used to describe any non-natural structure built on or affixed to land is "Improvement." Improvements refer to additions or modifications made to the land or property, which enhance its value or utility. This can include buildings, fences, roads, and other man-made structures that are permanently attached to the land.

In the context of real estate and property management, improvements are important as they can affect the land's overall valuation and legal status. Identifying something as an improvement helps to distinguish it from the natural land itself, thereby indicating a change or enhancement that has been made through human effort. This understanding is crucial for various applications, including property assessments, zoning regulations, and property investments.

Other terms like fixture or enhancement refer to specific types or aspects of improvements but do not encompass the broader range of non-natural structures associated with land. Therefore, "Improvement" is the most appropriate term for a non-natural structure affixed to land.