1. In computing, a user interface that provides access to the services provided by an operating system; also the name of a programming language.
2. A hard outer covering, e.g. of a nut or egg.
3. The protective body covering of a crustacean, e.g. lobster, crab.
4. The body covering of a turtle or tortoise (order Chelonia), reptiles with bodies enclosed in bony plates covered by horny scales to form an upper carapace, often fused to the vertebrae and ribs, and a lower plastron.