Something that, although detached, stands as part of another thing.
An attachment or appendage to something else.
Used often in a real estate context where an "appurtenance" may be, for example, a right-of-way over water, which, although physically detached, is part of the legal rights of the owner of another property.
In Cohen v Whitcomb, the Supreme Court of Minnesota (1919 142 Minn 20) adopted this definition of an appurtenance:
"That which belongs to something else. Something annexed to another thing more worthy."