The middle contact of a 3 conductor 1/4" (or mini 1/8") phone connector. Between the tip and the sleeve. Used in balanced wiring where the negative is connected to the ring, positive to the tip, and the shield to the sleeve. In stereo headphones, the tip carries the left signal, the ring carries the right signal, and the sleeve is the common (ground).
Sponsor: ISO 12231:2012 Standard