Aesculus

The genus Aesculus (/ˈaɪskjʊləs/) is a genus of flowering plants in the family Sapindaceae. They are trees and shrubs, mostly native to the temperate Northern Hemisphere, but one species (A. assamica) into subtropical areas of southeast Asia. The Plants of the World Online (POWO) database currently accepts 12 species, with seven species native to North America, four native to Asia, and one native to Europe; some other authors have accepted more species. Hardin (1957–1960) accepted 13 species, the Trees and Shrubs Online website cites "13–15" species with descriptions given for 14 species, and Harris et al. (2009) accepted 16 species. Six natural hybrids occur, and several others have been raised in cultivation. The European and Asian species are known as horse-chestnut or horse chestnut, and the North American species as buckeye. Aesculus exhibits a classical Arcto-Tertiary distribution.

Carl Linnaeus named the genus Aesculus after the Roman name for an edible acorn. The genus was considered to be in the ditypic family Hippocastanaceae along with Billia, but phylogenetic analyses of morphological and molecular data have more recently caused this family, along with the Aceraceae (maples and Dipteronia), to be included in the soapberry family (Sapindaceae). The fruit of the Mexican buckeye resemble Aesculus fruit, but belong to a related though different genus Ungnadia.

The common name "horse chestnut" invites confusion with the chestnuts in the genus Castanea in the order Fagales; the name is often hyphenated as horse-chestnut to minimise this confusion.