Flat design, 2010 to 2016: simple shapes, typography, solid colour, no gradients or strokes, rooted in the Swiss style. Metro in 2010, Holo in 2011, iOS 7 in 2013. The 45° long shadow was a fad of 2013. Material Design, announced June 2014, is flat plus a shadow: paper and ink, lit from above.
High-density screens made vector shapes scale cleanly and the responsive web punished textures. Nielsen called flat UIs a threat to usability and measured them slower. Material answered by putting depth back as elevation, so a button is a card that floats and a floating action button floats highest.
Six colours in the whole plate. Each slot of the figure is one solid colour, parts painted back to front, no strokes. The long shadow is the convex hull of each part together with its copy offset 220 pixels down and right, filled once at 15% and clipped to the disc. The red circle at the corner carries a two-layer elevation shadow.