Thyristor
From The Glossary of Digital Photography
A semiconductor switch or gate often used in electronic flash units to control the amount of light generated. The thyrsistor controls the light by allowing the energy from the capacitors to be sent to the flash tube to illuminate it when a control signal is received. Once the control signal stops, the thyristor gate is closed, stopping the energy from reaching the flash tube. This stops the flash from firing until the control signal returns.
