Most people use the phrase for any wall between two houses, and most of the time that is close enough. The Party Wall etc. Act 1996 is stricter, because the definition decides whether a notice has to be served before work starts, and a notice served on the wrong wall is as useless as no notice at all. There are really three things the Act calls a wall between owners, and telling them apart takes about a minute once you know what to look at.
The wall that stands astride the boundary
The clearest case is a wall built on the line between two properties, with part of it on each owner's land. In a Victorian terrace or a pair of semis this is normally the wall your neighbour's staircase runs up the other side of. Both owners have an interest in the whole of it, not just their half, which is why cutting into it, raising it or taking a chimney breast out of it is something the Act wants the other owner told about in advance. If the wall is astride the boundary and it separates buildings, you are almost certainly dealing with the Act.
The wall on one owner's land that still counts
A wall built entirely on one owner's land can still be a party wall for the Act's purposes if it separates buildings belonging to different owners. This catches a lot of real houses: an extension built up to the boundary, a garage wall the neighbour later built against, a flank wall that a later building leaned on. The test is what the wall does, which is separate two owners' buildings, rather than whose title deed the bricks sit inside. It is the case people miss, and it is the one that produces a notice nobody expected to serve.
The wall between them that is not a party wall at all
A garden wall that separates land rather than buildings is a party fence wall, not a party wall, and it has its own place in the Act. A wall wholly on your land that separates nothing but your own rooms is simply your wall, and you may do as you like with it, subject to planning and building control. And a fence, however solid, is not a wall. Getting this right is worth the minute it takes: serving a notice you did not owe wastes two months, and skipping one you did owe is the expensive mistake.
Questions people ask about party wall meaning
Is every wall between two houses a party wall?
No. It has to separate buildings in different ownership, or stand astride the boundary. A wall wholly inside your own building that touches nothing of the neighbour's is yours alone, whatever it looks like from the street.
Does the Act apply where I am?
Only in England and Wales. Scotland and Northern Ireland have no equivalent statute, and nothing on this site applies there. The same is true of the United States and anywhere else.
How do I tell where the boundary runs?
The Act does not decide boundaries and neither does this site. Title plans are indicative only, and a boundary argument is a matter for a solicitor rather than for a party wall notice. If the wall separates two owners' buildings, the Act applies whichever side of the line the bricks turn out to be.