A "Null Pointer Dereference" occurs when a program attempts to access or manipulate data through a pointer that has not been initialized or has been set to null. This can lead to unexpected behavior, crashes, or security vulnerabilities, as the program tries to read or write to a memory location that it shouldn't. In programming languages like C or C++, this error is common because pointers are used to reference memory locations. To prevent null pointer dereferences, developers should always check if a pointer is null before using it, ensuring that the program operates safely and correctly.