Definition of horse chestnut
1
: a large tree (Aesculus hippocastanum of the family Hippocastanaceae, the horse-chestnut family) of southeastern Europe that has palmate leaves and erect conical clusters of showy flowers and is widely cultivated as an ornamental and shade tree and naturalized as an escape;
also
: buckeye
2
: the large glossy brown seed of a horse chestnut
