The Collaborative International Dictionary of English v.0.48:

Flash \Flash\ (fl[a^]sh), v. i. [imp. & p. p. Flashed
   (fl[a^]sht); p. pr. & vb. n. Flashing.] [Cf. OE. flaskien,
   vlaskien to pour, sprinkle, dial. Sw. flasa to blaze, E.
   flush, flare.]
   1. To burst or break forth with a sudden and transient flood
      of flame and light; as, the lighting flashes vividly; the
      powder flashed.
      [1913 Webster]

   2. To break forth, as a sudden flood of light; to burst
      instantly and brightly on the sight; to show a momentary
      brilliancy; to come or pass like a flash.
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            Names which have flashed and thundered as the watch
            words of unnumbered struggles.        --Talfourd.
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            The object is made to flash upon the eye of the
            mind.                                 --M. Arnold.
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            A thought flashed through me, which I clothed in
            act.                                  --Tennyson.
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   3. To burst forth like a sudden flame; to break out
      violently; to rush hastily.
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            Every hour
            He flashes into one gross crime or other. --Shak.
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   flash in the pan, a failure or a poor performance,
      especially after a normal or auspicious start; also, a
      person whose initial performance appears augur success but
      who fails to achieve anything notable. From 4th pan, n.,
      sense 3 -- part of a flintlock. Occasionally, the powder
      in the pan of a flintlock would flash without conveying
      the fire to the charge, and the ball would fail to be
      discharged. Thus, a good or even spectacular beginning
      that eventually achieves little came to be called a flash
      in the pan.

   To flash in the pan, to fail of success, especially after a
      normal or auspicious start. [Colloq.] See under Flash, a
      burst of light. --Bartlett.
      [1913 Webster +PJC]

   Syn: Flash, Glitter, Gleam, Glisten, Glister.

   Usage: Flash differs from glitter and gleam, denoting a flood
          or wide extent of light. The latter words may express
          the issuing of light from a small object, or from a
          pencil of rays. Flash differs from other words, also,
          in denoting suddenness of appearance and
          disappearance. Flashing differs from exploding or
          disploding in not being accompanied with a loud
          report. To glisten, or glister, is to shine with a
          soft and fitful luster, as eyes suffused with tears,
          or flowers wet with dew.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Flash \Flash\, n.; pl. Flashes.
   1. A sudden burst of light; a flood of light instantaneously
      appearing and disappearing; a momentary blaze; as, a flash
      of lightning.
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   2. A sudden and brilliant burst, as of wit or genius; a
      momentary brightness or show.
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            The flash and outbreak of a fiery mind. --Shak.
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            No striking sentiment, no flash of fancy. --Wirt.
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   3. The time during which a flash is visible; an instant; a
      very brief period; as, I'll be back in a flash.
      [1913 Webster +PJC]

            The Persians and Macedonians had it for a flash.
                                                  --Bacon.
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   4. A preparation of capsicum, burnt sugar, etc., for coloring
      and giving a fictitious strength to liquors.
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   5. A lamp for providing intense momentary light to take a
      photograph; as, to take a picture without a flash.

   Syn: flashbulb, photoflash, flash lamp, flashgun.
        [WordNet 1.5]

   6. Same as flashlight. [informal]
      [PJC]

   7. (Journalism) A short news item providing recently received
      and usually preliminary information about an event that is
      considered important enough to interrupt normal
      broadcasting or other news delivery services; also called
      a news flash or bulletin.
      [PJC]

   Flash light, or Flashing light, a kind of light shown by
      lighthouses, produced by the revolution of reflectors, so
      as to show a flash of light every few seconds, alternating
      with periods of dimness. --Knight.

   Flash in the pan, the flashing of the priming in the pan of
      a flintlock musket without discharging the piece; hence,
      sudden, spasmodic effort that accomplishes nothing.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Pan \Pan\, n. [OE. panne, AS. panne; cf. D. pan, G. pfanne, OHG.
   pfanna, Icel., Sw., LL., & Ir. panna, of uncertain origin;
   cf. L. patina, E. paten.]
   1. A shallow, open dish or vessel, usually of metal, employed
      for many domestic uses, as for setting milk for cream, for
      frying or baking food, etc.; also employed for various
      uses in manufacturing. "A bowl or a pan." --Chaucer.
      [1913 Webster]

   2. (Manuf.) A closed vessel for boiling or evaporating. See
      Vacuum pan, under Vacuum.
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   3. The part of a flintlock which holds the priming.
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   4. The skull, considered as a vessel containing the brain;
      the upper part of the head; the brainpan; the cranium.
      --Chaucer.
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   5. (Carp.) A recess, or bed, for the leaf of a hinge.
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   6. The hard stratum of earth that lies below the soil. See
      Hard pan, under Hard.
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   7. A natural basin, containing salt or fresh water, or mud.
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   Flash in the pan. See under Flash.

   To savor of the pan, to suggest the process of cooking or
      burning; in a theological sense, to be heretical.
      --Ridley. Southey.
      [1913 Webster]

WordNet (r) 3.0 (2006):

flash in the pan
    n 1: someone who enjoys transient success but then fails

Moby Thesaurus II by Grady Ward, 1.0:

73 Moby Thesaurus words for "flash in the pan":
   abortion, abortive attempt, also-ran, bankrupt, best seller,
   big hit, blank cartridge, bogy, brilliant success, bubble, bugaboo,
   bugbear, castrato, cipher, cry of wolf, dud, dummy, ephemera,
   ephemeral, ephemerid, ephemerides, ephemeris, eunuch, fad, failure,
   false alarm, flop, gas, gasser, gelding, great success, hit,
   hollow man, impotent, incompetent, invalid, jackstraw, killing,
   lay figure, loser, man of straw, mayfly, merry chase,
   meteoric success, miscarriage, miscarrying, misfire,
   momentary success, nebbish, nobody, nonentity, nothing, nullity,
   puppet, pushover, resounding triumph, riot, roaring success,
   sensation, smash, smash hit, smoke, snows of yesteryear,
   thing of naught, trifle, triumph, vain attempt, washout, weakling,
   wet squib, wild-goose chase, wow, zero