CuO, or copper oxide, is a base because it can react with acids and neutralize them. The reaction and neutralization of the acid results in a salt and water. Bases are generally metal oxides or metal hydroxides due to their ability to react with acid.
CuO is a base, but it is not an alkali because it cannot be dissolved in water. If heated CuO is placed within an atmosphere of pure hydrogen, the products of the reaction are copper and water. CuO is black in color and is often used artistically for pigments and ceramics. It can also be found in batteries.