WordNet (r) 3.0 (2006):

split second
    n 1: a very short time (as the time it takes the eye to blink or
         the heart to beat); "if I had the chance I'd do it in a
         flash" [syn: blink of an eye, flash, heartbeat,
         instant, jiffy, split second, trice, twinkling,
         wink, New York minute]

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "split second":
   breath, breathing, coup, crack, flash, half a jiffy, half a mo,
   half a second, half a shake, instant, jiff, jiffy, microsecond,
   millisecond, minute, moment, sec, second, shake, stroke, tick,
   trice, twink, twinkle, twinkling, twitch, two shakes, wink