SIGHUP
The phone went dead — and daemons took it to mean “reread your configuration”.
The name is literal: hang up the telephone. The first terminals were teletypes hanging on phone lines, and when the call dropped the kernel warned that session's processes with this signal. The terminal is gone, the tower announces it: whoever depended on it wraps up.
Daemons — which have no terminal — inherited an idle bell and gave it a new ring: reread the configuration without stopping the service. nginx, sshd and friends reload when they hear it; and for the original effect there are still nohup and disown, which cover the process's ears before the window closes.
$ nginx -t
nginx: configuration file /etc/nginx/nginx.conf test is successful
$ sudo kill -HUP $(cat /run/nginx.pid)
(nginx rereads its configuration without dropping connections)