A moment so short it can barely be measured, like the time it takes to blink, is called a split second. A birdwatcher may catch sight of a hummingbird, only to have it disappear a split second later.
You can also call a split second a heartbeat or a flash. This term emphasizes how brief something is: "I only paused a split second before volunteering for the spelling bee." It can also be an adjective, as in "split second timing" or a "split second decision." The original split second was a type of stopwatch with two second hands, so that a fraction of a second could be timed.