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.
       [Webster 1913 Suppl.]

   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.
       [Webster 1913 Suppl.]

   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 file
    n 1: a file with two flat surfaces

The Free On-line Dictionary of Computing (30 December 2018):

flat file

    A single file containing flat
   ASCII representing or encoding some structure, e.g. a
   database, tree or network.

   Flat files can be processed with general purpose tools such as
   Perl and text editors but are less efficient than binary
   files if they must be parsed repeatedly by a program.  Flat
   files are more portable between different operating systems
   and application programs than binary files, and are more
   easily transmitted in electronic mail.

   See also flatten, sharchive.

   [Jargon File]

   (1996-01-26)