Walnut flour are the fruit of Juglans regia, which belongs to the family Juglandaceae.
From a botanical point of views, walnuts are drupes; they’re oily edible seeds enclosed in a wooden endocarp (the shell), which is covered by a fleshy layer (called husk, mesocarp and epicarp). From a dietary point of view, walnuts (whose seeds are the only edible part) belong to the group of dried fruit; instead, their husk can be used mainly as food colouring and flavouring.
Walnut flour is made from ripe seeds, which are removed from the fruit, then shucked and dried. They’re peeled and squeezed to extract oil (a quite refined product), while their fibrous remains are used to make walnut flour.