Party wall is a building term which, in England, apart from special statutory definitions, may be used in four different legal senses. It may mean

1. a wall of which the adjoining owners are tenants in common;
2. a wall divided longitudinally into two strips, one belonging to each of the neighboring owners;
3. a wall which belongs entirely to one of the adjoining owners, but is subject to an easement or right in the other to have it maintained as a dividing wall between the two tenements;
4. a wall divided longitudinally into two moieties, each moiety being subject to a cross easement, in favor of the owner of the other moiety.

The term can be used to describe a fireproof division between separate units in a condominium or housing complex.

References

Architectural elements | Architecture stubs