Stands for "Plastic Pin Grid Array."
PPGA is a processor package that uses a plastic backing to support the pin grid array, a series of metal pins on the underside of the chip that fit into a socket on the motherboard. It differs from other types of pin grid array packaging by its use of plastic instead of ceramic or other materials. The plastic is low-cost and dissipates heat quickly, allowing processors to use more transistors and run at higher speeds.
Intel used a PPGA packaging on several of its early Celeron processors. These CPU chips were produced between 1998 and 2000, and were popular for their power relative to Intel's more-expensive Pentium II series. They were also favored by overclockers, who found the chips to be stable when operated at faster speeds than Intel officially supported.
The PPGA packaging was introduced when Intel began moving away from the slot-based chips of the early Pentium II and Celeron series. PPGA itself fell out of use after Intel introduced the FCPGA (flip-chip pin grid array) packaging with the Pentium III series in 2000. Over time, pin grid array packaging was replaced entirely by LGA (land grid array) packaging, where the pins are instead located in the socket itself and connect to flat contacts on the underside of the chip.