The term 'device for measuring speed' can be summarized as a tachometer, which is a four-letter word. A tachometer is an instrument used to measure the rotational speed of a machine, such as an engine or motor. It is typically used to measure the revolutions per minute (RPM) of the engine, and can be used to ensure that the engine is running at its optimal speed. Tachometers are also used in other applications, such as measuring the speed of a vehicle or the speed of a wind turbine.