A tusk is an extremely long incisor tooth of certain mammals that protrudes when the mouth is closed. Tusk-bearing mammals include elephants, warthogs, wild boar, walruses, and narwhals. Tusks are used by humans to produce ivory, which is used in artifacts and jewelry, and formerly in other items such as piano keys.
Since most tusk-bearing mammals are endangered (especially the African elephant), the ivory trade is severely restricted by the United Nations Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). It is unknown if or when this restriction will end.