A mini plug is a 3.5mm (1/8 inch) plug that connects an audio or video device to an external output, such as speakers or headphones. Mini plugs are also used for microphone input. Mini plugs are technically called TRS connectors because they use three parts—the tip, the ring, and the sleeve (TRS)—to conduct sound. They are also called audio jacks, stereo jacks, stereo jack plugs, headphone jacks, jack plugs, and mini-jacks. Any variation of "plug" is more accurate than "jack," because "jack" usually refers to the socket the plug goes into.
The mini plug is a smaller version of the 6.35mm (1/4 inch) audio plug invented for telephone switchboards. Originally, the tip and the ring parts of the plug served different functions in telephone calls: The tip carried the sound of the telephone's bell and the voice, while the ring carried the busy signal. Telephone switchboards are largely a thing of the past, but both the larger plug and the mini plug have become standard for video and audio equipment. An even smaller 2.5mm plug, sometimes called a sub-mini plug, is often used with headphones, ear buds, and hands-free headsets for cell phones.
Depending on the number of conductors it has, a mini plug may deliver mono or stereo sound, or even video. Mono means that there is only one audio channel; the sound is the same in both speakers or both headphone earpieces. Mini plugs with two conductors carry a mono signal. Stereo means that there are two channels, so the sound in each speaker is different, but they are mixed so that the user hears them together. A stereo signal is carried by a three-conductor mini plug. Mini plugs with four conductors deliver a stereo signal, plus video.
Mini plugs are used for output with most modern audio devices, from portable audio players and DVD players to cell phones, computers, and stereos. One particularly interesting use is with workout software, which uses audio tones to control electronic exercise equipment such as treadmills. Mini plugs are used for input, via a microphone, with camcorders, computers, and audio recorders. Professional audio recording equipment is more likely to use 1/4 inch plugs than mini plugs, and high-end consumer equipment often uses the larger plugs as well. Either size plug can be used with the other size socket with the use of an adapter; these adapters are easy to find in most stores that carry home electronics.