Flash - A source of light that operates in a pulse. Electronic flash is now common, but the more old-fashioned expendable flash bulbs are still available and occasionally used. Modern cameras normally have only one type of synchronization, but on some older models there is a choice between 'X' and 'M'. 'X' is for electronic flash at all shutter speeds, and for flashbulbs and flashcubes at speeds slower than 1/30 second. 'M' synchronization works with flashbulbs and flashcubes at all shutter speeds.