Flyback
Flyback is a sudden high-voltage spike produced when the current through a coil or transformer is interrupted. The magnetic field stored in the coil collapses and pushes the voltage far above normal levels, sometimes to many times the supply voltage. These spikes can arc across insulation and damage transformers, switches or tubes.
Example: When a relay coil is switched off without a protective diode across it, the flyback spike can be large enough to damage the transistor that drives it.