The Collaborative International Dictionary of English v.0.48:

Instant \In"stant\, a. [L. instans, -antis, p. pr. of instare to
   stand upon, to press upon; pref. in- in, on + stare to stand:
   cf. F. instant. See Stand.]
   [1913 Webster]
   1. Pressing; urgent; importunate; earnest.
      [1913 Webster]

            Rejoicing in hope; patient in tribulation;
            continuing instant in prayer.         --Rom. xii.
                                                  12.
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            I am beginning to be very instant for some sort of
            occupation.                           --Carlyle.
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   2. Closely pressing or impending in respect to time; not
      deferred; immediate; without delay.
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            Impending death is thine, and instant doom. --Prior.
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   3. Present; current.
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            The instant time is always the fittest time.
                                                  --Fuller.
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   Note: The word in this sense is now used only in dates, to
         indicate the current month; as, the tenth of July
         instant.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Instant \In"stant\, adv.
   Instantly. [Poetic]
   [1913 Webster]

         Instant he flew with hospitable haste.   --Pope.
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The Collaborative International Dictionary of English v.0.48:

Instant \In"stant\, n. [F. instant, fr. L. instans standing by,
   being near, present. See Instant, a.]
   [1913 Webster]
   1. A point in time; a moment; a portion of time too short to
      be estimated; also, any particular moment; as, the
      situation may change in an instant.
      [1913 Webster +PJC]

            There is scarce an instant between their flourishing
            and their not being.                  --Hooker.
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   2. A day of the present or current month; as, the sixth
      instant; -- an elliptical expression equivalent to the
      sixth of the month instant, i. e., the current month. See
      Instant, a., 3.

   Syn: Moment; flash; second.
        [1913 Webster]

WordNet (r) 3.0 (2006):

instant
    adj 1: occurring with no delay; "relief was instantaneous";
           "instant gratification" [syn: instantaneous,
           instant(a)]
    2: in or of the present month; "your letter of the 10th inst"
       [syn: instant, inst]
    3: demanding attention; "clamant needs"; "a crying need";
       "regarded literary questions as exigent and momentous"-
       H.L.Mencken; "insistent hunger"; "an instant need" [syn:
       clamant, crying, exigent, insistent, instant]
    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]
    2: a particular point in time; "the moment he arrived the party
       began" [syn: moment, minute, second, instant]