"Fluent" was the official name for the new Office UI system introduced with Office 2007 (which included not only the ribbon, but other stuff like the new contex

"Fluent" was the official name for the new Office UI system introduced with Office 2007 (which included not only the ribbon, but other stuff like the new context menus/floating toolbar, new keyboard shortcut system, etc.) The name didn't really catch on and I think Microsoft stopped using it after a short time, but I guess they eventually reused it for the new design concept introduced with Windows Redstone.