split-second

noun2

1. •
   A fraction of a second; hence (figurative, also attributive) a very short time period; an instant, a moment.

   • It's very dangerous, though, when a man thinks for a split second that he understands death. — 1912
2. •
   [before a noun] A stopwatch with two second hands, each of which may be stopped separately to time the intervals of a race by a sportsperson, or to time more than one sportsperson.