Flyback diode
also known as: Freewheeling diode · Inductive kickback · Back EMF
A diode placed backwards across an inductive load (relay coil, motor, solenoid) to catch the voltage spike at turn-off. An inductor's current can't stop instantly — cut its path and the voltage slams up hunting for one; the diode is the path.
In practice
Without it, a 12 V relay coil can put hundreds of volts on the switching transistor's collector — the classic silent microcontroller killer. The diode does nothing while the load runs (it's reverse-biased) and conducts only during the kick. 1N4007-class is fine for relays.