In very simple terms, a wall that divides two properties.
If the wall stands over (astride) the boundary between two different properties, it will be a party wall.
If the wall stands entirely upon one side of a boundary between two different properties, but the wall still separates buildings belonging to two different owners, it will be a party wall.
The Party Wall etc. Act 1996 sets out the definition of what constitutes a party wall in further detail.
If you plan works that involve a party wall in some way you should seek advice from a specialist party wall surveyor and/or a party wall solicitor to ensure that you are following the correct procedures and protocols to avoid any action being taken against you.
If you believe your neighbour is carrying out works to a party wall that you share and you have not been properly consulted you should seek advice from a specialist party wall surveyor and/or a party wall solicitor to advise you of your rights.
Child & Child have a number of specialist party wall solicitors with wide experience of court proceedings involving party wall disputes, from numerous trials of domestic and commercial party wall disputes in the county court to complex cases involving trials in the Chancery Division of the High Court and appeals to the Court of Appeal.