Tap harmonic is a technique used with fretted string instruments, (usually guitar

Guitar

The guitar is a plucked string instrument, usually played with fingers or a pick. The guitar consists of a body with a rigid neck to which the strings, generally six in number, are attached. Guitars are traditionally constructed of various woods and strung with animal gut or, more recently, with...

). It is executed by tapping

Tapping

Tapping is a guitar playing technique, where a string is fretted and set into vibration as part of a single motion of being pushed onto the fretboard, as opposed to the standard technique being fretted with one hand and picked with the other...

on the actual fret wire most commonly at the 12th fret, but also can be executed by tapping any of the fret wires with proper technique. It can also be done by gently touching the string over the fret wire instead of tapping the fret wire if the string is already ringing.