What term describes an illegal wall or fence built on another person's property?

The term that describes an illegal wall or fence built on another person's property is "encroachment." This refers to a situation where a property owner builds a structure, such as a wall or fence, that extends beyond their property line into the property of a neighbor. Encroachments can lead to disputes and may require legal resolutions, especially if the encroaching structure negatively affects the neighboring property owner’s rights or use of their land.

Easement typically refers to a legal right to use another person's land for a specific purpose, such as a right-of-way, and does not imply an illegal installation. Adverse possession, on the other hand, involves a person gaining legal ownership of land by occupying it without the owner's permission for a certain period, but does not directly relate to the act of building an illegal structure. A boundary dispute refers to disagreements between property owners about the location of property lines but does not specifically address the erection of a wall or fence.