What is the full form of XC6SLX9?
It's a part number - it doesn't have to mean anything, and to the best of my knowledge, Xilinx haven't given a detailed explanation of what this part number means. They do explain some of the other parts of a complete device marking, as seen below (from the Spartan-6 datasheet), but the base part number is treated as a single entity.



That being said, we can draw some inferences from some of the part numbers of Xilinx's other products. The "LX" in Spartan-6 parts is vestigal; it's an artifact of some other FPGA families which have multiple sub-families, such as the Virtex line which has the three "LXT", "SXT", and "HXT" sub-families which have slightly different features and functionality. The Spartan-6 has no such subfamilies, so all Spartan-6 parts are "LX" parts by default.

The number following that seems to be an approximation of the number of logic cells in the part, divided by 1000. For instance, the XC6SLX9 has 9,152 logic cells, and the XC6SLX100T has 101,261 logic cells. This feature is stable across FPGA families, so it can be used to compare parts in one line with another. For instance, the Artix-7 XC7A35T is roughly intermediate in size between the Spartan-6 XC6SLX25 and XC6SLX45.