Emacs revert-buff (s-u): what is s-u?

Question

The emacs help page for the M-x revert-buffer command says that it is bound to "s-u". What the heck is s-u? I know M-x is Meta-x (usually alt-x), and I know that C-x is Control-x. But what is s-u?

Answer 1

On Mac OS X 10.6.8 (Snow Leopard) with Emacs: 23.3.1, s-u corresponds to Cmd-u.