What does "corundum" mean?

Corundum: The earth alumina, as found native in a crystalline state, including sapphire, which is the fine blue variety; the oriental ruby, or red sapphire; the oriental amethyst, or purple sapphire; and adamantine spar, the hair-brown variety. It is the hardest substance found native, next to the diamond. Note: The name corundum is sometimes restricted to the non- transparent or coarser kinds. Emery is a dark-colored granular variety, usually admixed with magnetic iron ore.