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.
      [1913 Webster]

            I am beginning to be very instant for some sort of
            occupation.                           --Carlyle.
      [1913 Webster]

   2. Closely pressing or impending in respect to time; not
      deferred; immediate; without delay.
      [1913 Webster]

            Impending death is thine, and instant doom. --Prior.
      [1913 Webster]

   3. Present; current.
      [1913 Webster]

            The instant time is always the fittest time.
                                                  --Fuller.
      [1913 Webster]

   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.
   [1913 Webster]

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.
      [1913 Webster]

   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]

Moby Thesaurus II by Grady Ward, 1.0:

190 Moby Thesaurus words for "instant":
   a breath, about to be, actual, acute, alert, already in sight,
   approaching, apt, as is, at hand, being, bit, breath, breathing,
   brewing, burning, clamant, clamorous, close, close at hand, coming,
   compelling, contemporaneous, contemporary, coup, crack, critical,
   crucial, crying, current, cursory, day, decisive, demanding, dire,
   direct, draining, earnest, exacting, exigent, existent, existing,
   exorbitant, expeditious, extant, extortionate, fateful moment,
   festinate, feverish, flash, flying, forthcoming, fresh, furious,
   future, gathering, going to happen, grasping, hair-trigger,
   half a jiffy, half a mo, half a second, half a shake, hasty,
   high-pressure, high-priority, hour, hurried, immanent, immediate,
   imminent, impendent, impending, imperative, imperious, importunate,
   in danger imminent, in prospect, in reserve, in store,
   in the cards, in the offing, in the wind, in view, insistent,
   instantaneous, interval, jiff, jiffy, juncture, kairos,
   last-minute, latest, lightning-like, little, little bit,
   little while, looming, loud, lowering, lurking, menacing,
   microsecond, millisecond, minute, modern, moment, moment of truth,
   momentaneous, momentary, near, near at hand, nearing, new, no time,
   on the horizon, on the spot, overhanging, overnight, pair of winks,
   passing, period, persistent, pertinacious, pivotal, point,
   pregnant moment, prepared, preparing, present, present-age,
   present-day, present-time, pressing, presto, prompt,
   psychological moment, punctual, quick, quick as lightning,
   quick as thought, ready, ready-made, ready-mixed, running, season,
   sec, second, shake, short spell, short time, slap-bang, slapdash,
   small space, snap, space, span, speedy, spell, split second, spurt,
   stage, stretch, stroke, summary, superficial, swift, taxing,
   that be, that is, that will be, threatening, tick, time, time lag,
   to come, topical, trice, twink, twinkle, twinkling, twitch,
   two shakes, up-to-date, up-to-the-minute, upcoming, urgent,
   waiting, while, wink