A "hard shell" refers to a protective outer layer that is rigid and durable. This term is commonly used in various contexts, such as in biology to describe the tough exterior of certain animals like tortoises and crabs. These shells serve as a defense mechanism against predators and environmental hazards. In technology, "hard shell" can describe protective cases for devices like smartphones and laptops. These cases are designed to absorb impact and prevent damage from drops or scratches, ensuring the longevity of the devices they encase.