Redline refers to the maximum engine speed at which an internal combustion engine or traction motor and its components are designed to operate without causing damage to the components themselves or other parts of the engine. The redline of an engine depends on various factors such as stroke, mass of the components, displacement, composition of components, and balance of components.
Tachometer showing red lines above 14,000 rpm.
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 (RPM) on a calibrated analogue dial, but digital displays are increasingly common.
A tachometer that can indicate up to 7000 RPM (left)
Cessna 172's G1000 tachometer (1,060 RPM) and engine hours (1736.7 hours)
A tachometer on a tractor, reading up to 3000 RPM, with the hour meter below it showing 772.9 hours. The mark at 2500 RPM is the engine speed required to run the power take-off at 540 RPM.