Single In-Line Pin Package
The last "standard" PC RAM configuration before they started making SIMMs. A lot like SIMMs, but they have little pins instead of contacts. SIPPs are to VLB what SIMMs are to PCI.
Truth be known later SIPPs were actually SIMMs with little pins soldered onto the contacts. SIPPs could be made into SIMMs by simply melting off the little pins. An astounding 3 out of 10 of these modified SIPPs didn't suffer terminal chip damage from the heat.
They also had an interesting habit of leaping off the motherboard after repeated heating and cooling.
Even with all their oddness they were a lot better than the other weird proprietary vendor specific stuff they had out at the time. It was also better than the good old individual chips on the XTs.