split-second

Short definition from Wiktionary

Noun

1. 1
   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.
2. 2
   A fraction of a second; hence (figurative, also attributive) a very short time period; an instant, a moment.
3. 3
   Alternative form of split-second.