A "21D tempered glass" is a screen protector designed with aggressive marketing terms, not a scientifically distinct type of glass. It's essentially a high-quality, multi-layered screen protector made of 9H hardness tempered glass, featuring a 3D (or "21D") curved edge for full coverage and a smooth, natural feel on the phone's display. Key features include scratch and shock resistance, a smooth surface, and an oil/fingerprint-resistant coating.