The Collaborative International Dictionary of English v.0.48:

Jiffy \Jif"fy\, n. [Perh. corrupt. fr. gliff.] [Written also
   giffy.]
   A moment; an instant; as, I will be ready in a jiffy.
   [Colloq.] --J. & H. Smith.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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



The Jargon File (version 4.4.7, 29 Dec 2003):

jiffy
 n.

    1. The duration of one tick of the system clock on your computer (see tick
    ). Often one AC cycle time (1/60 second in the U.S. and Canada, 1/50 most
    other places), but more recently 1/100 sec has become common. ?The swapper
    runs every 6 jiffies? means that the virtual memory management routine is
    executed once for every 6 ticks of the clock, or about ten times a second.

    2. Confusingly, the term is sometimes also used for a 1-millisecond wall
    time interval.

    3. Even more confusingly, physicists semi-jokingly use ?jiffy? to mean the
    time required for light to travel one foot in a vacuum, which turns out to
    be close to one nanosecond. Other physicists use the term for the
    quantum-nechanical lower bound on meaningful time lengths,

    4. Indeterminate time from a few seconds to forever. ?I'll do it in a jiffy
    ? means certainly not now and possibly never. This is a bit contrary to the
    more widespread use of the word. Oppose nano. See also Real Soon Now.


The Free On-line Dictionary of Computing (30 December 2018):

jiffy

   1. The duration of one tick of the computer's system
   clock.  Often one AC cycle time (1/60 second in the US and
   Canada, 1/50 most other places), but more recently 1/100 sec
   has become common.

   2. Confusingly, the term is sometimes also used for a
   1-millisecond wall time interval.  Even more confusingly,
   physicists semi-jokingly use "jiffy" to mean the time required
   for light to travel one foot in a vacuum, which turns out to
   be close to one *nanosecond*.

   [Jargon File]

   (2002-03-02)