An interface in programming, particularly in Java, is a reference type that defines a contract for classes to implement. It can contain constants, method signatures, default methods, static methods, and nested types, but it does not provide method implementations for most methods. Interfaces are crucial for achieving full abstraction, allowing different classes to interact without needing to know the specifics of each other’s implementations. This promotes a modular design, enabling developers to work independently on different components of a system. Interfaces also facilitate multiple inheritance-like behavior in Java, as a class can implement multiple interfaces, enhancing code reusability and flexibility.