In music, a drone is a harmonic or monophonic effect or accompaniment where a note or chord is continuously sounded throughout much or all of a piece, sustained or repeated, and most often establishing a tonality upon which the rest of the piece is built. The systematic (not occasional) use of drones originated in Ancient Southwest Asia and spread north and west to Europe, east to India, and south to Africa (van der Merwe 1989, p.11). It is used in Indian music and is played by the tambura.

(Stolen from http://en.wikipedia.org/w iki/Drone_%28music%29)