This was originally editorial issue #6185.

The term "undefined address" is used in [atomics.types.pointer]/8 and [atomics.ref.pointer]/6, however, it seems not properly defined and the intended meaning is unclear. The term originally appeared in the paper N2393 and was reused by P0019R8, but no explanation can be found in these papers.

Perhaps "undefined address" is related to the undefined behavior specified in [expr.add].