Source : Webster's Revised Unabridged Dictionary (1913)

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

         Instant he flew with hospitable haste.   --Pope.

Instant \In"stant\, n. [F. instant, fr. L. instans standing by,
   being near, present. See {Instant}, a.]
   1. A point in duration; a moment; a portion of time too short
      to be estimated; also, any particular moment.

            There is scarce an instant between their flourishing
            and their not being.                  --Hooker.

   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.

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. in?tant. See {Stand}.]
   1. Pressing; urgent; importunate; earnest.

            Rejoicing in hope; patient in tribulation;
            continuing instant in prayer.         --Rom. xii.
                                                  12.

            I am beginning to be very instant for some sort of
            occupation.                           --Carlyle.

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

            Impending death is thine, and instant doom. --Prior.

   3. Present; current.

            The instant time is always the fittest time.
                                                  --Fuller.

   Note: The word in this sense is now used only in dates, to
         indicate the current month; as, the tenth of July
         instant.

Source : WordNet®

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: {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 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}, {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}]