A jack plug is a common audio connector. It is cylindrical in shape, with two or more contacts. It was invented for use in telephone switchboards in the 19th century and is still widely used, both in its original 6.3mm size and in miniaturized versions.

Modern jack plugs are available in three standard sizes. The original 6.3 mm version dates from 1878, for use in manual telephone exchanges—making it possibly the oldest electrical connector standard still in use. The 3.5 mm size was originally designed as a two-conductor connector for earpieces on transistor radios. Both sizes are now readily available in two-conductor (mono) and three-conductor (stereo or tip ring sleeve) versions.