Flash duration
From The Glossary of Digital Photography
The length of time that the light from a flash is active above a certain threshold. Short flash durations are important to stop motion in some photographic situations. Typical flash durations in camera-mounted flashes are around 1/1,000th of a second. With auto flash settings, the flash is cut off once the amount of light needed is reached so flash durations can be shorter, in the range of 1/20,000th of a second for subjects that are closer to the auto flash sensor.
