SIGTERM
On *n?x systems, a signal sent to programs to "ask" them to terminate, or end. Unlike SIGKILL, SIGTERM allows the program to do clean-up work (e.g. save, close, and delete files) before it ends.
During the shutdown process, processes are sent SIGTERM, and if there are some that don't want to terminate, those are sent SIGKILL a few seconds afterward.