Walnut is the fruit of the walnut tree in genus Juglans. The nut of all the species are edible, but the walnuts most commonly available in shops are from the Persian walnut (Juglans regia). Persian walnuts are the only species which has a large nut and thin shell. The nuts are rich in oil, and are widely eaten both fresh and in cookery. The nuts are also pressed to obtain walnut oil, which is expensive and used sparingly in salad dressing.
Walnut is an excellent source of omega-3 fatty acids, and have been known in lowering cholesterol. Due to its high content of unsaturated fat, it needs to be kept dry and refrigerated to prevent it from becoming rancid, especially in warm conditions.