Butternut \But"ter*nut`\, n. 1. (Bot.) An American tree ({Juglans cinerea}) of the Walnut family, and its edible fruit; -- so called from the oil contained in the latter. Sometimes called {oil nut} and {white walnut}. 2. (Bot.) The nut of the {Caryocar butyrosum} and {C. nuciferum}, of S. America; -- called also {Souari nut}.