Communication signals sent from the OS to a process. Whenever these signals are sent to the process, the program must halt and handle this signal with its custom-defined handler function Signal List https://www.computerhope.com/unix/signals.htm SIGHUP SIGINT SIGQUIT SIGILL SIGTRAP SIGABRT SIGEMT SIGFPE SIGKILL SIGBUS SIGSEGV SIGSYS SIGPIPE SIGALRM SIGTERM SIGURG SIGSTOP SIGTSTP SIGCONT SIGCHLD