The Collaborative International Dictionary
Horse-chestnut \Horse`-chest"nut\, Horsechestnut \Horse`chest"nut\, n.
(Bot.) The large nutlike seed of a species of [AE]sculus ( [AE]sculus Hippocastanum), formerly ground, and fed to horses, whence the name. The seed is not considered edible by humans. [WordNet sense 2]
(Bot.) The tree itself ( Aesculus hippocastanum), which was brought from Constantinople in the beginning of the sixteenth century, and is now common in the temperate zones of both hemispheres; it has palmate leaves and large clusters of white to red flowers followed by brown shiny inedible seeds. The native American species is also called buckeye and conker. [WordNet sense 1]
Esculin \Es*cu"lin\, n. [See Esculic.] (Chem.) A glucoside obtained from the [AE]sculus hippocastanum, or horse-chestnut, and characterized by its fine blue fluorescent solutions. [Written also [ae]sculin.]
Wikipedia
Aesculus hippocastanum is a species of flowering plant in the soapberry and lychee family Sapindaceae. It is a large deciduous, synoecious tree, commonly known as horse-chestnut or conker tree.