An integument is an outer layer, like a human's skin or a walnut's shell.
Use the noun integument when you need a scientific term for an organism's protective covering. Lizards' scales are their integument, and the germ or husk of a seed is its integument. Tissues and coverings inside animals, such as the protective membranes around organs, are another kind of integument. The Latin root is integumentum, "a covering," from integere, "to cover over."