1. Any tuning aid that can be used to set an instrument to a specific pitch. These include things like a tuning fork, pitch pipe, and various electronic tuners. 2. A thumbscrew or similar mechanism attached to a peg at one end of many stringed instruments used to regulate the tension of individual strings for setting their pitch, i.e. tuning them. 3. An electronic device designed to receive and demodulate modulated electromagnetic signals such as those generated by radio and television transmitters. For example a radio tuner is designed to be able to “tune in to” and pick up a variety of nearby transmitting radio stations.