`std::bad_cast` is an exception class in C++ that is part of the Standard Library. It is thrown when a dynamic cast fails to convert a pointer or reference to a base class into a pointer or reference to a derived class. This typically occurs when the object being cast does not actually belong to the derived class type. When a `std::bad_cast` exception is thrown, it indicates that the type conversion is invalid. This exception can be caught using a try-catch block, allowing programmers to handle the error gracefully. It is important to use dynamic casting carefully to avoid runtime errors in C++ applications.