Backside cache is used by Apple Computers, Inc. in their PowerPC G3 processor. Previous PowerPC processors used the system bus to access both secondary cache and main memory. In the PowerPC G3 a dedicated bus handles only CPU/cache transactions. This bus can operate faster than the system bus thus improving the overall performance of the processor.
The term apparently derives from the relocation of the secondary cache from the motherboard to the processor card itself, i.e. on the backside of the processor card.