in 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.

a ˌsplit ˈsecond

a very short time: I heard a loud explosion and a split second later I was on the floor. ▶ ˌsplit-ˈsecond adj.: split-second timing/reactions