What does 'Si' mean with respect to network switches?

Solution 1:

It means Silicon Switch Processor a high-performance 7000 series router.

According to the answer from dtlokee in this Cisco forum, it does stand for SSP.

They use the Si for the "silicon switching", I guess it's a bit dated but that is how they would represent a switch doing L3 switching in hardware. Now I think it goes without saying that multilayer switching is done in hardware. They also used a switch icon with an extra bit on the bottom that had 4 rectangles with lines between them to represent a L3 switch with the MFFC installed (going back to the 5000/5500 here)