The Collaborative International Dictionary of English v.0.48:

Flat \Flat\ (fl[a^]t), a. [Compar. Flatter (fl[a^]t"r[~e]r);
   superl. Flattest (fl[a^]t"t[e^]st).] [Akin to Icel. flatr,
   Sw. flat, Dan. flad, OHG. flaz, and AS. flet floor, G.
   fl["o]tz stratum, layer.]
   1. Having an even and horizontal surface, or nearly so,
      without prominences or depressions; level without
      inclination; plane.
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            Though sun and moon
            Were in the flat sea sunk.            --Milton.
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   2. Lying at full length, or spread out, upon the ground;
      level with the ground or earth; prostrate; as, to lie flat
      on the ground; hence, fallen; laid low; ruined; destroyed.
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            What ruins kingdoms, and lays cities flat! --Milton.
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            I feel . . . my hopes all flat.       --Milton.
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   3. (Fine Arts) Wanting relief; destitute of variety; without
      points of prominence and striking interest.
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            A large part of the work is, to me, very flat.
                                                  --Coleridge.
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   4. Tasteless; stale; vapid; insipid; dead; as, fruit or drink
      flat to the taste.
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   5. Unanimated; dull; uninteresting; without point or spirit;
      monotonous; as, a flat speech or composition.
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            How weary, stale, flat, and unprofitable
            Seem to me all the uses of this world. --Shak.
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   6. Lacking liveliness of commercial exchange and dealings;
      depressed; dull; as, the market is flat.
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   7. Clear; unmistakable; peremptory; absolute; positive;
      downright.

   Syn: flat-out.
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              Flat burglary as ever was committed. --Shak.
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              A great tobacco taker too, -- that's flat.
                                                  --Marston.
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   8. (Mus.)
      (a) Below the true pitch; hence, as applied to intervals,
          minor, or lower by a half step; as, a flat seventh; A
          flat.
      (b) Not sharp or shrill; not acute; as, a flat sound.
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   9. (Phonetics) Sonant; vocal; -- applied to any one of the
      sonant or vocal consonants, as distinguished from a
      nonsonant (or sharp) consonant.
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   10. (Golf) Having a head at a very obtuse angle to the shaft;
       -- said of a club.
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   11. (Gram.) Not having an inflectional ending or sign, as a
       noun used as an adjective, or an adjective as an adverb,
       without the addition of a formative suffix, or an
       infinitive without the sign to. Many flat adverbs, as in
       run fast, buy cheap, are from AS. adverbs in -["e], the
       loss of this ending having made them like the adjectives.
       Some having forms in ly, such as exceeding, wonderful,
       true, are now archaic.
       [Webster 1913 Suppl.]

   12. (Hort.) Flattening at the ends; -- said of certain
       fruits.
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   Flat arch. (Arch.) See under Arch, n., 2. (b).

   Flat cap, cap paper, not folded. See under Paper.

   Flat chasing, in fine art metal working, a mode of
      ornamenting silverware, etc., producing figures by dots
      and lines made with a punching tool. --Knight.

   Flat chisel, a sculptor's chisel for smoothing.

   Flat file, a file wider than its thickness, and of
      rectangular section. See File.

   Flat nail, a small, sharp-pointed, wrought nail, with a
      flat, thin head, larger than a tack. --Knight.

   Flat paper, paper which has not been folded.

   Flat rail, a railroad rail consisting of a simple flat bar
      spiked to a longitudinal sleeper.

   Flat rods (Mining), horizontal or inclined connecting rods,
      for transmitting motion to pump rods at a distance.
      --Raymond.

   Flat rope, a rope made by plaiting instead of twisting;
      gasket; sennit.

   Note: Some flat hoisting ropes, as for mining shafts, are
         made by sewing together a number of ropes, making a
         wide, flat band. --Knight.

   Flat space. (Geom.) See Euclidian space.

   Flat stitch, the process of wood engraving. [Obs.] -- Flat
   tint (Painting), a coat of water color of one uniform shade.


   To fall flat (Fig.), to produce no effect; to fail in the
      intended effect; as, his speech fell flat.
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            Of all who fell by saber or by shot,
            Not one fell half so flat as Walter Scott. --Lord
                                                  Erskine.
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WordNet (r) 3.0 (2006):

flat arch
    n 1: an arch with mutually supporting voussoirs that has a
         straight horizontal extrados and intrados [syn: flat
         arch, straight arch]