From Longman Dictionary of Contemporary Englishtuning forkˈtuning fork noun [countable] 
APMa small U-shaped steel instrument that makes a particular musical note when you hit it
APMa small U-shaped steel instrument that makes a particular musical note when you hit itExamples from the Corpus
tuning fork• A gravitational wave at the natural frequency for longitudinal oscillations of the bar would set it ringing like a tuning fork.• Didymograptus species of this type have a distinctive shape like a tuning fork.• Delaney's bomb had shaken through the ship, vibrating the steel like a tuning fork.• I trembled like a tuning fork, but my shoulder fakes absorbed the worst of the shaking. 