a split second

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.
See also: second, split

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
See also: second, split