Stands for "Scalable Link Interface." SLI is a technology developed by NVIDIA that allows multiple graphics cards to work together in a single computer system. This enables faster graphics performance than what is possible with a single card. For example, using SLI to link two cards together may offer up to twice the performance of a single video card. If each card has two GPUs, the result may be up to four times the performance of a typical video card!
For video cards to be linked using NVIDIA's SLI system, the computer must have multiple PCI Express slots. PCI Express is the first video card interface that allows the linking of multiple graphics cards because the slots share the same bus. Previous technologies, such as PCI and AGP, used separate buses, which did not allow graphics cards to be bridged together. The PCI Express slots must also support enough bandwidth for the cards, which typically means they must be x8 or x16 slots. Of course, the cards themselves must also support SLI bridging in order to work together.