In the context of
photography, a
flash (or
strobe) is a device that sends out a very short lasting and very powerful flash of light.
The effect is achieved either by an electrical discharge (more or less like a spark) in a rare gas (usually xenon), or by igniting some fast-burning material, like magnesium in air or steel wool in pure oxygen atmosphere: the first case is the electronic flash, and the latter is the flashbulb.