When a person builds an improvement that extends onto the property of a neighbor, it is considered a(n):

The correct answer is understanding the concept of encroachment, which refers to the situation where a property owner builds or extends a structure onto their neighbor's land without permission. This can happen with fences, buildings, or other improvements and is considered an unlawful intrusion. Encroachments can lead to disputes between property owners and might require legal action to resolve.

In this scenario, encroachment highlights the importance of respecting property boundaries and the consequences of violating those lines. It's crucial for property owners to be aware of their boundaries to avoid potential legal issues and the need for resolution between neighbors.

Other concepts in the choices relate to different legal situations; for example, defeasance deals with the termination of rights or obligations, injunctions are court orders to prohibit certain actions, and easements give someone the right to use another person's property for a specific purpose. While these terms are important in property law, they do not accurately describe the scenario of building on a neighbor's property.