SLI was coined by 3DFX, and it stood for scan-line interleve, and involved two PCI graphics cards. This technology was pioneered with the Voodoo2 3D Accelerator graphics cards. SLI meant that one card would handle the even-numbered polygons, and the other card would handle the odd-numbered polygons. Theoretically, this would lead to a near-doubling of frame rates, but performance gains were usually more modest.
NVIDIA's implementation is slightly different, and SLI here does not mean anything beyond the acronym itself. Previously, NVIDIA cards needed to be identical for SLI to work (to share computational power between up to four PCI Express graphics cards), but cards now will downclock to the slowest card in the range used.