a split second

In an instant; in less than a full second of time. Yes, I lost my leg in a car accident. In a split second, my whole life changed. It was heartbreaking to see the entire building reduced to rubble in a split second. Cherish the moments with those you love and be thankful for everything you have, because tragedy can happen in a split second.

Not even a full second of time; an instant. Hyphenated if used as a modifier before a noun. There was only a split second separating the top two competitors in the race. It was a split-second decision that undoubtedly saved hundreds of lives. It was heartbreaking to see the entire building reduced to rubble in a split second.