Tempered or toughened glass is a type of safety glass processed by controlled thermal or chemical treatments to increase its strength compared with normal glass.

Tempering puts the outer surfaces into compression and the inner surfaces into tension.
Such stresses cause the glass, when broken, to crumble into small granular chunks instead of splintering into jagged shards as plate glass (aka: annealed glass) creates.
The granular chunks are less likely to cause injury.