What is the name of the design language introduced in Windows 10?

The design language introduced in Windows 10 was the Fluent Design System.

Microsoft announced Fluent Design in 2017 under the earlier codename Project Neon. It was presented as a cross-platform design system for Windows and Microsoft software, with an emphasis on light, depth, motion, material, and scale. Its visual vocabulary included translucency, blur, layered surfaces, animation, and adaptive layouts.

Fluent began appearing in Windows 10 with the Fall Creators Update, released in October 2017. It was not the original name for Windows 10's interface: Metro, later called Microsoft Design Language, described the flatter visual approach associated with Windows 8 and early Windows 10. Aero was the glass-like design language most strongly associated with Windows Vista and Windows 7.

Fluent continued into Windows 11 and related Microsoft platforms. Modern Fluent materials include Acrylic and Mica, although their use varies by operating-system version, app framework, and interface surface.