I agree with
@Aibohphobia that DC-ATX is a class that encompasses separate boards, but 'Direct plug' doesn't immediately imply a PSU integrated into the plug, just that something plugs in directly, i.e. without an adapter. As the DC-DC PSU itself could be thought of as an adapter, the name could be confusing for an average reader.
How about Plug Integrated DC, or
PIDC? Incorporating 'DC' into the acronym immediately informs laymen that "this item does something with power, but not AC wall power", and 'PI' does not have any alternate uses in the PC realm, so gives the acronym an immediately unique identifiable nature, even without knowledge of what the acronym stands for.
'Plug integrated' I'm a bit less happy with as it could imply integration into a plug at either 'end' of the DC-to-ATX adapter e.g. integration of the DC in jack as with the HDPlex's DC-ATX boards that mount where an ATX PSU would. Even though that is technically a socket there is room for an interpretation as "a DC thing integrated into that bit I plug into".