Transient Voltage Suppressors

Transient Voltage Suppressors are specialised zener diodes which clamp the voltage across a line to prevent transients from  damaging sensitive electronics.
The clamping occurs when the voltage across the line exceeds the zener avalanche rating. The clamping is more precise than with a Varistor but the energy
handling capability tends not to be as high.

On telecommunications and data systems, and other special circuits, TVS's can be combined very effectively with Gas Discharge Tubes in "hybrid" circuits
to get high current handling combined with low voltage clamping.