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 (about 7-35 inches), 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

• Sect. Juglans. Leaves large (20-45 cm) with 5-9 broad leaflets, hairless , margins entire. Wood hard. Southeast Europe to central Asia.
• Juglans regia L. (J. duclouxiana Dode, J. fallax Dode, J. orientis Dode) - Persian Walnut, Carpathian, or Common Walnut
• Juglans 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.
• Juglans australis Griseb. (J. boliviana Dode) - Argentine Walnut
• Juglans brasiliensis Dode - Brazilian Walnut
• Juglans californica S.Wats. - California Walnut
• Juglans hindsii (Jepson) R.E.Smith - Hinds' Walnut
• Juglans hirsuta Manning - Nuevo Leon Walnut
• Juglans jamaicensis C.DC. (J. insularis Griseb.) - West Indies Walnut
• Juglans major (Torrey) Heller (J. arizonica Dode, J. elaeopyron Dode, J. torreyi Dode) - Arizona Walnut
• Juglans major var. glabrata Manning
• Juglans microcarpa Berlandier (J. rupestris Engelm.) - Texas Walnut or Little Walnut
• Juglans microcarpa var. stewartii (Johnston) Manning
• Juglans mollis Engelm. - Mexican Walnut
• Juglans neotropica Diels (J. honorei Dode) - Andean Walnut
• Juglans nigra L. - Black Walnut
• Juglans olanchana Standl. & L.O.Williams -
• Juglans peruviana Dode - Peruvian Walnut
• Juglans soratensis Manning -
• Juglans steyermarkii Manning - Guatemalan Walnut
• Juglans venezuelensis Manning - Venezuela Walnut
• Sect. Cardiocaryon. Leaves very large (40-90 cm) with 11-19 broad leaflets, softly downy, margins serrated. Wood soft. Northeast Asia, eastern North America.
• Juglans ailantifolia Carr. (J. cordiformis Maxim., J. sieboldiana Maxim.) - Japanese Walnut
• Juglans cinerea L. - Butternut
• Juglans mandshurica Maxim. (J. cathayensis Dode, J. formosana Hayata, J. hopeiensis Dode, J. stenocarpa Maxim.) - Manchurian Walnut or Chinese Walnut.