Parent Case
The standardized cartridge case that another cartridge or wildcat is derived from by necking, shortening, or reshaping the brass.
A parent case is the established cartridge whose brass forms the starting point for a new design. A derived round might neck the parent up or down for a different bullet, shorten it for a compact action, or blow out the shoulder for more case capacity. Knowing the parent is practical information, because it tells the handloader which brass to buy and reform and which dies and shellholder will fit.
Many modern factory cartridges share well-known parents, and the lineage explains a lot about their behavior. The .308 Winchester, for example, is the parent of a large family that includes the 7mm-08 and the .243. An Ackley Improved chambering or any other wildcat is defined almost entirely by what its parent case is and how it was altered.