The skin, also known as rind or peel, is the outer protective layer of a fruit or vegetable that can be peeled off. The skin is usually the botanical exocarp, but the term exocarp also includes the hard cases of nuts, which are not named skin or peels since they are not peeled off by hand or peeler, but rather shells because of their hardness. A fruit with a thick peel, such as a citrus fruit, is called a hesperidium. In hesperidia, the inner layer (also called albedo or, among non-botanists, pith) is peeled off together with the outer layer (called flavedo), and together they are called the peel. The flavedo and albedo, respectively, are the exocarp and the mesocarp. The juicy layer inside the peel (containing the seeds) is the endocarp.

Source: Wikipedia