Emacs恢复-buff (s-u):什么是s-u?

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?The emacs help page for the M-x revert-buffer c