SIS stands for Shimano Index System (or something like that) but is used interchangeably with index shifting.

Indexing shifting is where you pull or push on the shifter until it 'clicks' and the rear derailleur moves or indexes the chain onto the selected cog.

Friction shifting is where you manually move and adjust the chain onto the selected cog and then trim the derailleur.

Accushift is Suntours name for it, as Mentioned Shimano coined SIS, and Campagnolos first attempt was called Syncro, but that sort of a hybrid system where is clicked but required a little over shift and sometimes some trimming.

Shimanos move to SIS, SLR (Shimano Leaner Respones) and STI (Shimano Total Integration aka brifters) was as much about forcing bike builders/sellers to use all or no shimano. derailleurs of course would only index with shimano shifters but they also tried to make the brakes feel spongey and weak if you didn't use shimano brake levers and calipers together. OOOPs sorry I'll stop shipmano bashing now.