In music, a drone is a harmonic or monophonic effect or accompaniment where a note or chord is continuously sounded throughout most or all of a piece. The word drone is also used to refer to any part of a musical instrument that is just used to produce such an effect.
A drone effect can be achieved through a sustained sound or through repetition of a note. It most often establishes a tonality upon which the rest of the piece is built. A drone can be instrumental, vocal or both.