Acronym for "Pocket Dimension Zone". They are the 'places within places', magically sealed zones that look similar to what exists on the actual world, but in a underlying dimension untouched by the forces outside. These areas are immensely difficult to build unnaturally, requiring countless years of rune building and magical seals by some of the most powerful Spellweaver around. And even then, the 'natural world' must accept the PDZ, or all the progress will begin to break down into Wild Magics, becoming unusable.
Natural PDZ, from which the unnatural ones were inspired by, were almost entirely made by ancient, bygone civilizations and the Fey. Only a handful of Spellweavers in the history of magic have discovered the designs of such areas, and only the most powerful have copied them.