Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

jiffy
     n : a very short time (as the time it takes the eye 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},
          {split second}, {trice}, {twinkling}, {wink}, {New York
         minute}]

Source : Free On-Line Dictionary of Computing

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)