Processor Direct Slot
The Processor Direct Slot or PDS, introduced by Apple Computer, in several of their Macintosh models, provided a limited measure of hardware expandability, without going to the expense (in terms of desk space as well as selling price) of providing full-fledged bus expansion slots.

Typically, a machine would feature multiple bus expansions slots, if any. However, there was never more than one PDS slot. Rather than providing a sophisticated communication protocol with arbitration between different bits of hardware that might be trying to use the communication channel at the same time, the PDS slot, for the most part, just gave direct access to signal pins on the CPU, making it closer in nature to a local bus.

Thus, PDS slots tended to be CPU-specific, and therefore a card designed for the PDS slot in the Motorola 68030-based Macintosh SE/30, for example, would not work in the Motorola 68040-based Quadra 700.

The one notable exception to this was the PDS design for the original Motorola 68020-based Macintosh LC. This was Apple's first attempt at a "low-cost" Mac, and it was such a success that, when subsequent models replaced the CPU with a 68030, a 68040, and later a PowerPC processor, Apple found methods to keep the PDS slot compatible with the original LC, so that the same expansion cards would continue to work.

External links
 Pictures of several PDS cards at AppleFritter
 What kind of expansion or upgrade slots does my Mac have? at Mac FAQ
 Notes on PDS variations at Apple Wiki

Macintosh internals
Motherboard expansion slot