chestnut, name for any species of the genus Castanea, deciduous trees of the family Fagaceae (beech or oak family) widely distributed in the Northern Hemisphere. They are characterized by thin-shelled, sweet, edible nuts borne in a bristly bur. The common American chestnut, C. dentata, is native E of the Mississippi but is now nearly extinct because of the chestnut blight, a disease from Asia caused by the fungus Crypthonectria parasitica, and the clear-cutting that resulted when lumber companies anticipated the destruction of chestnut forests by the fungus. The American chestnut was an important source of timber. Efforts have been made to breed a type of American chestnut resistant to the disease, by crossing it with the blight-resistant Chinese and Japanese chestnuts, in order to replace the old chestnut forests, and significant plantings of the largely American hybrids have been made. The dead and fallen logs were long the the leading domestic source of tannin. Chestnut wood is porous, but it is very durable in soil and has been popular for fence posts, railway ties, and beams. Edible chestnuts are now mostly imported from Italy, where the Eurasian species ( C. sativa ) has not been destroyed. The chinquapin belongs to the same genus. Chestnuts are classified in the division Magnoliophyta, class Magnoliopsida, order Fagales, family Fagaceae.