In computing, control-\ is a control character in ASCII code, also known as the file separator (FS) character. It is generated by pressing the \ key while holding down the Ctrl key on a computer keyboard.
Under most UNIX based operating systems control-\ is used to terminate a running process from a command shell and have it produce a memory core dump by sending it a SIGQUIT signal.
In the Emacs text editor, it is the default keystroke mapping for autocompletion.
References
Control-\ Wikipedia(Text) CC BY-SA