Phone plug   

Standard audio connector. 

Comes in stereo, tip/sleeve/ring and mono versions. Also in several diameters - 1/4, 1/8 (also known as “mini”) and 3/16th (or “Bantam”).  Bantam plugs are used exclusively in patch bays - often those built-in to large mixing consoles. 1/8” plugs are usually found in consumer devices or very small units.  Tip/sleeve/ring plugs (bottom right) are used for balanced connections in patch bays and are available in 1/4" and Bantam. T/S/R plugs are different from stereo plugs - the "ring" is not the same length. 

1/4" phone plugs are commonly used in instruments (electric guitars, synthesizers, etc.) signal processors (reverbs, delays, compressors),  for line level signals in mixers and amplifiers and generally throughout professional and semi-professional audio worlds for unbalanced signals. Aside from professional headphones, which use 1/4" stereo plugs,  most balanced connections are made with xlr connectors or T/S/R.