A timer or auto timer was a type of clock that when set, indicated a specific amount of time had passed. It could be used to trigger a function to happen automatically. The term time factor referred to the amount a time remained on a time until said time expired.
In 2154, Malcolm Reed noted that a bomb's timer was broken, but the explosive itself was armed. (ENT: "The Forge")
USS Enterprise's transporter had a timer. In 2269, Montgomery Scott set the timer on the transporters to a 10 minute automatic return for James T. Kirk when the captain beamed down to Terra 10. (TAS: "The Terratin Incident")
The time-pod stolen by Berlinghoff Rasmussen had an auto timer which would take its occupants to 22nd century New Jersey. (TNG: "A Matter Of Time")