_Parallel Line Internet Protocol_
It's like SLIP, but it goes over a NULL Printer/LapLink/Plip cable. You can connect your antiquated laptop to your PC through the parallel port where your printer would usually go.
There are two configurations that you can use, you can either cross over the data out and data in lines or you can connect data out to the status return lines.
Poor old PLIP isn't finding all that many uses these days due to the low price, speed, etc of ethernet cards.