Ready light
From The Glossary of Digital Photography
A light on the back of a flash or other electronic device that signifies when the device is ready to use. A ready light attached to a flash is important because the capacitors take time to recharge before the flash can fire. In a battery-operated flash, as the batteries get weaker the recharging time can grow from a few seconds to many seconds. The ready light allows the photographer to wait until there is enough energy to fire the flash properly. (see figure on page 259)
