noun
tachometer; revolution counter
Technical term for a device that measures rotational speed, typically in RPM.
エンジンの回転計が急に動かなくなった。
The engine's tachometer suddenly stopped working.
回転計でモーターの回転数を測る。
Measure the motor's rotation speed with a tachometer.
タコメーター is the common loanword equivalent, used in everyday contexts like car dashboards.
Compound of 回転 (rotation) and 計 (meter, gauge).