What does "AH00169: caught SIGTERM, shutting down" mean in the Apache error log?

Question

When parsing the /var/log/apache2/error_log file during an investigation, you find the following error.

AH00169: caught SIGTERM, shutting down

What does this mean and why does it happen?

Answer

This means that the Apache service was restarted.  SIGTERM is used to restart the Apache service as long as it is set up to auto-restart in initd, which it is on cPanel servers.  There are various reasons why Apache may gracefully restart without manual intervention, as highlighted in the following article

Why does Apache do graceful restarts?

Otherwise, you may want to check to see if the server was rebooted, as Apache will be shut down as part of the reboot process as well.