A TIMER file is a unit configuration file included with systemd, an init (initialization) system used by various Linux distributions to bootstrap user space and manage processes. It contains information about a timer that is controlled and supervised by systemd, which includes the type of timer and the service that is activated when the timer elapses.

systemd init system is a suite of programs included in various Linux distributions. The system is used to manage different aspects of a server. A TIMER file contains information about when a service is activated. A service is contained in a .SERVICE file. For each TIMER file, there is a corresponding SERVICE file that describes the unit that is activated when the timer finishes. Also, the files bear the same name. For example, a example.timer file corresponds with the example.service file. NOTE: systemd is booted by many Linux distributions, including Red Hat, Arch Linux, Fedora, CentOS, Ubuntu, Mageia, CoreOS, and Alpine Linux.