Walnuts (genus Juglans) are plants in the family Juglandaceae. They are deciduous trees, 10–40 meters tall (about 30–130 ft), with pinnate leaves 200–900 millimetres long (7–35 in), with 5–25 leaflets; the shoots have chambered pith, a character shared with the wingnuts (Pterocarya) but not the hickories (Carya) in the same family.

The 21 species in the genus range across the north temperate Old World from southeast Europe east to Japan, and more widely in the New World from southeast Canada west to California and south to Argentina. The Latin name, Juglans, derives from Jovis glans, "Jupiter's acorn": figuratively, a nut fit for a god.

The word walnut derives from Old English wealhhnutu, literally "foreign nut", wealh meaning "foreign" (wealh is akin to the terms Welsh and Vlach; see *Walha and History of the term Vlach).Online Etymology Dictionary - "Walnut" The walnut was so called because it was introduced from Gaul and Italy. The previous Latin name for the walnut was nux Gallica, "Gallic nut".

Species and classification

The genus Juglans is divided into four sections:

• Sect. Juglans. Leaves large (20–45 cm) with 5–9 broad leaflets, hairless, margins entire. Wood hard. Southeast Europe to central Asia.
• J. regia L. (J. duclouxiana Dode, J. fallax Dode, J. orientis Dode) — common walnut, Persian, English, or Carpathian walnut
• J. sigillata Dode — Iron Walnut (doubtfully distinct from J. regia)
• Sect. Rhysocaryon. Leaves large (20–50 cm) with 11–23 slender leaflets, finely pubescent, margins serrated. Wood hard. North America, South America.
• J. australis Griseb. (J. boliviana Dode) — Argentine Walnut
• J. boliviana (C. DC.) Dode — Bolivian walnut, Peruvian walnut
• J. brasiliensis Dode — Brazilian Walnut
• J. californica S.Wats. — California Black Walnut
• J. hindsii (Jepson) R.E.Smith — Hinds' Black Walnut
• J. hirsuta Manning — Nuevo Leon Walnut
• J. jamaicensis C.DC. (J. insularis Griseb.) — West Indies Walnut
• J. major (Torrey) Heller (J. arizonica Dode, J. elaeopyron Dode, J. torreyi Dode) — Arizona Black Walnut
• J. major var. glabrata Manning
• J. microcarpa Berlandier (J. rupestris Engelm.) — Texas Walnut or Little Black Walnut
• J. microcarpa var. stewartii (Johnston) Manning
• J. mollis Engelm. — Mexican Walnut
• J. neotropica Diels (J. honorei Dode) — Andean Walnut, Cedro Negro , Cedro Nogal , Nogal , Nogal Bogotano
• J. nigra L. — Eastern Black Walnut
• J. olanchana Standl. & L.O.Williams — Cedro Negro, Nogal , Walnut
• J. peruviana Dode — Peruvian Walnut
• J. soratensis Manning
• J. steyermarkii Manning — Guatemalan Walnut
• J. venezuelensis Manning — Venezuela Walnut
• Sect. Cardiocaryon. Leaves very large (40–90 cm) with 11–19 broad leaflets, softly downy, margins serrated. Wood soft. Fruits borne in racemes of up to 20. Nuts have thick shells. Northeast Asia.
• J. ailantifolia Carr. (J. cordiformis Maxim., J. sieboldiana Maxim.) — Japanese Walnut
• J. ailantifolia var. cordiformis — Heartnut
• J. mandshurica Maxim. (J. cathayensis Dode, J. formosana Hayata, J. hopeiensis Dode, J. stenocarpa Maxim.) — Manchurian Walnut or Chinese Walnut.
• Sect. Trachycaryon. Leaves very large (40–90 cm) with 11–19 broad leaflets, softly downy, margins serrated. Wood soft. Fruits borne in clusters of 2-3. Nuts have a thick, rough shell bearing distinct, sharp ridges. Eastern North America.
• J. cinerea L. — Butternut