Definition - What does Party Wall mean?
Used in the context of real estate and related fields, party wall is the legal name given to a shared barrier that is on a property line between two neighboring residential or commercial buildings. As such, it is sometimes, but not always, built as the result of a contractual agreement between the property owners.
Justipedia explains Party Wall
Because party walls are built to benefit both property owners, both are responsible for maintenance costs (even if the wall itself is mostly or entirely on one person's land).
As a rule, a property owner owns the portion of the wall on their land. Barring stipulations to the contrary, they also have the right to use the portion of the wall belonging to the other person.
Finally, party wall agreements are associated with the property; they generally apply to anyone who takes ownership of said property in the future.