1. An outer protective covering; such as, the feathers or skin of an animal, a shell, or a rind (hard, tough outer layer, particularly on food; such as, fruit, cheese, etc.).
2. The outer layer of an ovule of a plant which develops into the seed coat.
3. Something which covers or encloses; especially, an enveloping layer (as a skin, membrane, or cuticle) of an organism or one of its parts.