A tachometer is an instrument which measures the motor speed. The word's roots are Greek: tachos, meaning speed, and metron, meaning measure. The traditional tachometer is laid out as a dial, with a needle indicating the current reading and marking safe and dangerous levels.
Prolonged use at high speeds may cause excessive wear and other damage to motors. On an analog tachometer this maximum speed is typically indicated by an area of the gauge marked in red, giving rise to the expression of "redlining" an engine - i.e. running it at (dangerously) high speed.