split second

an instant; a tiny period of time. (*Typically: for ~; in ~.) The lightning struck, and in a split second the house burst into flames. For a split second, it looked like she would fall.

An instant, a fraction of a second, as in Our best swimmer came in a split second before theirs. This expression alludes to a stop watch that has two second hands, one above the other, for timing more than one athlete or intervals of a race by a single athlete. Each hand can be stopped independently of the other, so a second can be "split" when one second hand stops a fraction of a second after the other. [c. 1880]