3.5 mm

Somewhat confusingly, 3.5mm connectors are often called “mini jack” connectors – even the plugs. The type is indeed a smaller and space-saving version of the larger phone connector (6.35mm), and has an outer diameter of 3.5mm. The connector is often called 1/8", although that “conversion” isn’t really correct (1/8 inch is 3.175mm).

Mini-jack connectors are mainly used in headphone jacks on computers, mobile phones, media players and so on. They are also used for other audio signals with line level, for example, on system cameras or in modular synthesizers such as Eurorack and similar equipment with limited space. 3.5mm connectors are also used for smaller condenser microphones, for example in portable recording equipment. In these cases, stereo connectors are often used, with one conductor for the microphone's signal, and one conductor for phantom power to the microphone.
Mini jack connectors are cheap and available in many forms with different features, and ready-made cables in several different lengths are easy to come by. Thus, they can be a good choice even also for non-audio electronics, and they have indeed been used for low-quality (composite) video and DC power supplies.

3.5mm connectors are available with two, three or four poles. One of the conductors is always used for ground, and it is always the one furthest back on the plug. It is known as the sleeve, or S for short. The sleeve is also connected to the connector housing (unless plastic). The other contacts are known as tip (T) and ring (R), the latter of which there may be one or two of.