Segmentation fault segfault
Posix signal SIGSEGV — the program tried to access memory it isn't allowed to read/write. Most common cause of crashes in C/C++. NULL dereference, use-after-free, buffer overflow.
The kernel detects it via MMU page fault → sends SIGSEGV → default handler crashes the process and produces a core dump. The message Segmentation fault (core dumped) is Unix iconic. Debugging: gdb on the core file, AddressSanitizer (-fsanitize=address) in build, Valgrind. Modern languages (Rust, Go, JS, Python) eliminate it almost by design — ownership/GC/runtime checks. C++ smart pointers reduce the risk.