SLI stands for scalable link interface. This is a graphics technology coined by Nvidia that allows your PC to run two Nvidia SLI-branded GPUs (aka graphics cards aka video cards) simultaneously. To do so, you’ll also need an SLI-certified motherboard and an SLI connector.
In Nvidia’s own words: “NVIDIA SLI intelligently scales graphics performance by combining multiple Georce GTX GPUs on an SLI-certified motherboard. … SLI features an intelligent communication protocol embedded in the GPU, a high-speed digital interface to facilitate data flow between the two graphics cards and a complete software suite providing dynamic load balancing, advanced rendering and compositing to ensure maximum compatibility and performance in today’s latest games.”
Nvidia currently has a long list of SLI-capable GPUs.
This article is part of the Tom's Hardware Glossary.