Describes a type of plug most-typically used to connect MIDI interfaces. The DIN stands for 'Deutsches Institut für Normung' (German Industry for Standards).
The plug has a round metal shield with a notch inside the bottom (to assist proper alignment) along with an array of five equally spaced pins in an arc towards the top (all inside the round shield ring).
The standard five pin DIN MIDI cable has served many a musician connecting simple MIDI controllers to entire racks of gear. They are inexpensive, reliable (most anyway), and easily obtainable.