Source : Webster's Revised Unabridged Dictionary (1913)

Flat \Flat\, a.
   1. (Golf) Having a head at a very obtuse angle to the shaft;
      -- said of a club.

   2. (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.

   3. (Hort.) Flattening at the ends; -- said of certain fruits.

Flat \Flat\, v. t. [imp. & p. p. {Flatted}; p. pr. & vb. n.
   {Flatting}.]
   1. To make flat; to flatten; to level.

   2. To render dull, insipid, or spiritless; to depress.

            Passions are allayed, appetites are flatted.
                                                  --Barrow.

   3. To depress in tone, as a musical note; especially, to
      lower in pitch by half a tone.

Flat \Flat\, v. i.
   1. To become flat, or flattened; to sink or fall to an even
      surface. --Sir W. Temple.

   2. (Mus.) To fall form the pitch.

   {To flat out}, to fail from a promising beginning; to make a
      bad ending; to disappoint expectations. [Colloq.]

Flat \Flat\, a. [Compar. {Flatter}; superl. {Flattest}.] [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.

            Though sun and moon Were in the flat sea sunk.
                                                  --Milton.

   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.

            What ruins kingdoms, and lays cities flat! --Milton.

            I feel . . . my hopes all flat.       --Milton.

   3. (Fine Arts) Wanting relief; destitute of variety; without
      points of prominence and striking interest.

            A large part of the work is, to me, very flat.
                                                  --Coleridge.

   4. Tasteless; stale; vapid; insipid; dead; as, fruit or drink
      flat to the taste.

   5. Unanimated; dull; uninteresting; without point or spirit;
      monotonous; as, a flat speech or composition.

            How weary, stale, flat, and unprofitable Seem to me
            all the uses of this world.           --Shak.

   6. Lacking liveliness of commercial exchange and dealings;
      depressed; dull; as, the market is flat.

   7. Clear; unmistakable; peremptory; absolute; positive;
      downright.

            Flat burglary as ever was committed.  --Shak.

            A great tobacco taker too, -- that's flat.
                                                  --Marston.

   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.

   9. (Phonetics) Sonant; vocal; -- applied to any one of the
      sonant or vocal consonants, as distinguished from a
      nonsonant (or sharp) consonant.

   {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.

            Of all who fell by saber or by shot, Not one fell
            half so flat as Walter Scott.         --Lord
                                                  Erskine.

Flat \Flat\, adv.
   1. In a flat manner; directly; flatly.

            Sin is flat opposite to the Almighty. --Herbert.

   2. (Stock Exchange) Without allowance for accrued interest.
      [Broker's Cant]

Flat \Flat\, n.
   1. A level surface, without elevation, relief, or
      prominences; an extended plain; specifically, in the
      United States, a level tract along the along the banks of
      a river; as, the Mohawk Flats.

            Envy is as the sunbeams that beat hotter upon a
            bank, or steep rising ground, than upon a flat.
                                                  --Bacon.

   2. A level tract lying at little depth below the surface of
      water, or alternately covered and left bare by the tide; a
      shoal; a shallow; a strand.

            Half my power, this night Passing these flats, are
            taken by the tide.                    --Shak.

   3. Something broad and flat in form; as:
      (a) A flat-bottomed boat, without keel, and of small
          draught.
      (b) A straw hat, broad-brimmed and low-crowned.
      (c) (Railroad Mach.) A car without a roof, the body of
          which is a platform without sides; a platform car.
      (d) A platform on wheel, upon which emblematic designs,
          etc., are carried in processions.

   4. The flat part, or side, of anything; as, the broad side of
      a blade, as distinguished from its edge.

   5. (Arch.) A floor, loft, or story in a building; especially,
      a floor of a house, which forms a complete residence in
      itself.

Source : WordNet®

flat
     adj 1: having a horizontal surface in which no part is higher or
            lower than another; "a flat desk"; "acres of level
            farmland"; "a plane surface" [syn: {level}, {plane}]
     2: having no depth or thickness
     3: not modified or restricted by reservations; "a categorical
        denial"; "a flat refusal" [syn: {categoric}, {categorical},
         {unconditional}]
     4: stretched out and lying at full length along the ground;
        "found himself lying flat on the floor" [syn: {prostrate}]
     5: lacking contrast or shading between tones [ant: {contrasty}]
     6: lowered in pitch by one chromatic semitone; "B flat" [ant: {natural},
         {sharp}]
     7: flattened laterally along the whole length (e.g., certain
        leafstalks or flatfishes) [syn: {compressed}]
     8: lacking taste or flavor or tang; "a bland diet"; "insipid
        hospital food"; "flavorless supermarket tomatoes"; "vapid
        beer"; "vapid tea" [syn: {bland}, {flavorless}, {flavourless},
         {insipid}, {savorless}, {savourless}, {vapid}]
     9: lacking stimulating characteristics; uninteresting; "a bland
        little drama"; "a flat joke" [syn: {bland}]
     10: having lost effervescence; "flat beer"; "a flat cola"
     11: not increasing as the amount taxed increases [syn: {fixed}]
     12: not made with leavening; "most flat breads are made from
         unleavened dough" [syn: {unraised}]
     13: parallel to the ground; "a flat roof"
     14: without pleats [syn: {unpleated}]
     15: lacking the expected range or depth; not designed to give an
         illusion or depth; "a film with two-dimensional
         characters"; "a flat two-dimensional painting" [syn: {two-dimensional}]
     16: (of a tire) completely or partially deflated
     17: not reflecting light; not glossy; "flat wall paint"; "a
         photograph with a matte finish" [syn: {mat}, {matt}, {matte},
          {matted}]
     18: lacking variety in shading; "a flat unshaded painting"
     [also: {flatting}, {flatted}, {flattest}, {flatter}]

flat
     adv 1: at full length; "he fell flat on his face"
     2: with flat sails; "sail flat against the wind"
     3: below the proper pitch; "she sang flat last night"
     4: against a flat surface; "he lay flat on his back"
     5: in a forthright manner; candidly or frankly; "he didn't
        answer directly"; "told me straight out"; "came out flat
        for less work and more pay" [syn: {directly}, {straight}]
        [ant: {indirectly}]
     6: wholly or completely; "He is flat broke"
     [also: {flatting}, {flatted}, {flattest}, {flatter}]

flat
     n 1: a level tract of land
     2: a shallow box in which seedlings are started
     3: a musical notation indicating one half step lower than the
        note named
     4: freight car without permanent sides or roof [syn: {flatcar},
         {flatbed}]
     5: a deflated pneumatic tire [syn: {flat tire}]
     6: scenery consisting of a wooden frame covered with painted
        canvas; part of a stage setting
     7: a suite of rooms usually on one floor of an apartment house
        [syn: {apartment}]
     [also: {flatting}, {flatted}, {flattest}, {flatter}]

Source : Free On-Line Dictionary of Computing

flat
     
        1. Lacking any complex internal structure.  "That {bitty box}
        has only a flat file system, not a hierarchical one."  The verb
        form is {flatten}.  Usually used pejoratively (at least with
        respect to file systems).
     
        2. Said of a memory architecture like that of the {VAX} or
        {Motorola} {680x0} that is one big linear address space
        (typically with each possible value of a processor register
        corresponding to a unique address).  This is a {Good Thing}.
        The opposite is a "{segmented}" architecture like that of the
        {Intel 80x86} in which addresses are composed from a
        base-register/offset pair.  Segmented designs are generally
        considered cretinous.
     
        3. A flat {domain} is one where all elements except {bottom}
        are incomparable (equally well defined).  E.g. the integers.
     
        [{Jargon File}]