An electronic device that analyzes the frequency of the note an instrument is playing, and indicates whether that frequency is sharp or flat compared to a reference pitch (usually standard pitch, although most electronic tuners can also be calibrated to use other frequencies as their reference pitch).
An electronic tuner may acquire the incoming signal in a variety of ways: a built-in or external microphone, a direct connection using a cable (as with a guitar or bass output), or using a transducer that attaches to the instrument and senses its vibration.
Electronic tuners range from very simple devices that are limited to just displaying the six notes used for standard guitar tuning, to chromatic tuners that can display any note in equal temperament, to tuners that can be set up to display various “sweetened” or modified tunings for special applications and/or intonation needs.