Design Nomenclature / Form & Geometry
Undercut
Also called negative draft, back-draft, die lock
A region of a form that cannot be withdrawn along a given direction: any face whose normal turns back past the pull, so the mould locks onto the part, or that the cutter cannot see from where it must approach. It is a property of the form relative to a direction, not of the form alone.
In practice
Change the pull and the undercut moves or disappears, which is why the argument is about the parting line first and the form second. Where the form has to keep it, the price is a side action, a split or flexible mould, another axis, or a process that never pulls at all.