Fluent Design System

Had head this term being used recently.  In Engadget:

Microsoft's design rules push Windows 'beyond mere rectangles'
Exploring the core tenets of the Fluent Design System

by Sean Buckley, @seaniccus

Microsoft's Fall Creators Update for Windows 10 might have an ironically uncreative name, but the upgrade itself is flush with artistic potential and useful features. It will give users a timeline to manage complex work sessions, APIs that tie all of Microsoft's services together and, notably, a new design paradigm intended to radically overhaul the flat-rectangle user interface it's known for. Microsoft's Fluent Design System focuses on five tenets to help developers build more creative and engaging user interfaces: depth, material, light, scale and motion..... "