Ever notice how quick the flash is on a camera? That's how fast other types of flashes are, including the super-fast superhero The Flash.
Whether you're talking about a flash of lighting or a flash of insight, a flash is quick and bright and sudden. There aren't any slow, long flashes. Because flashes are so fast, the word is often used for anything that happens really quickly: a flash is as fast as a New York minute, the blink of an eye, a heartbeat, an instant, a jiffy, or a split second.