Wiktionary
n. The creative short-circuiting of electronic devices such as children's toys and small digital synthesizers in order to create new musical or visual instruments.
Wikipedia
Circuit bending is the creative, chance-based customization of the circuits within electronic devices such as low voltage, battery-powered guitar effects, children's toys and digital synthesizers to create new musical or visual instruments and sound generators.
Emphasizing spontaneity and randomness, the techniques of circuit bending have been commonly associated with noise music, though many more conventional contemporary musicians and musical groups have been known to experiment with "bent" instruments. Circuit bending usually involves dismantling the machine and adding components such as switches and potentiometers that alter the circuit.