The Tach Timer is an instrument used in aviation

Aviation

Aviation is the design, development, production, operation, and use of aircraft, especially heavier-than-air aircraft. Aviation is derived from avis, the Latin word for bird.-History:...

 to accumulate the total number of revolutions performed by the engine. The unit of measure is equivalent to the number of hours of running at a certain, specific reference speed of rotation. If the reference speed of rotation is 2400 RPM then the Tach Timer records elapsed real (i.e. clock) time when the engine is running at 2400 RPM. However, the timer will run at half clock speed while the engine is run at 1200 RPM (a fast idle for some aviation engines) or at 5/6ths real time at 2000 RPM (a slow cruise speed). So the Tach Timer operates slower than clock time when the engine is operating at low speeds of rotation and faster than clock time at higher speeds of rotation.
The tach timer integrates over time the instantaneous rotation speed displayed by the tachometer

Tachometer

A tachometer is an instrument measuring the rotation speed of a shaft or disk, as in a motor or other machine. The device usually displays the revolutions per minute on a calibrated analogue dial, but digital displays are increasingly common...

. The displayed number is incremented by one if the engine is run at its reference speed for one hour. The quantity recorded is referred to as tach(ometer) hours. If the reference rotation speed is 2400 RPM then the tach timer records

Uses

The tach timer is usually used to schedule engine maintenance. It can also be used as a basis for charging for aircraft rental as opposed to charging for actual elapsed time. This encourages the renter to properly warm the engine before take off and also not to run the engine continuously at maximum speed.