NOUN (1)

1. cultivated land that is not seeded for one or more growing seasons;


ADJECTIVE (2)

1. left unplowed and unseeded during a growing season;
- Example: "fallow farmland"

2. undeveloped but potentially useful;
- Example: "a fallow gold market"

The Collaborative International Dictionary of English v.0.48:

Fallow \Fal"low\, n. [So called from the fallow, or somewhat
   yellow, color of naked ground; or perh. akin to E. felly, n.,
   cf. MHG. valgen to plow up, OHG. felga felly, harrow.]
   1. Plowed land. [Obs.]
      [1913 Webster]

            Who . . . pricketh his blind horse over the fallows.
                                                  --Chaucer.
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   2. Land that has lain a year or more untilled or unseeded;
      land plowed without being sowed for the season.
      [1913 Webster]

            The plowing of fallows is a benefit to land.
                                                  --Mortimer.
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   3. The plowing or tilling of land, without sowing it for a
      season; as, summer fallow, properly conducted, has ever
      been found a sure method of destroying weeds.
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            Be a complete summer fallow, land is rendered tender
            and mellow. The fallow gives it a better tilth than
            can be given by a fallow crop.        --Sinclair.
      [1913 Webster]

   Fallow crop, the crop taken from a green fallow. [Eng.]

   Green fallow, fallow whereby land is rendered mellow and
      clean from weeds, by cultivating some green crop, as
      turnips, potatoes, etc. [Eng.]
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Fallow \Fal"low\, a. [AS. fealu, fealo, pale yellow or red; akin
   to D. vaal fallow, faded, OHG. falo, G. falb, fahl, Icel.
   f["o]lr, and prob. to Lith. palvas, OSlav. plav[u^] white, L.
   pallidus pale, pallere to be pale, Gr. polio`s gray, Skr.
   palita. Cf. Pale, Favel, a., Favor.]
   1. Pale red or pale yellow; as, a fallow deer or greyhound.
      --Shak.
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   2. [Cf. Fallow, n.] Left untilled or unsowed after plowing;
      uncultivated; as, fallow ground.
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   Fallow chat, Fallow finch (Zool.), a small European bird,
      the wheatear (Saxicola [oe]nanthe). See Wheatear.
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The Collaborative International Dictionary of English v.0.48:

Fallow \Fal"low\, v. t. [imp. & p. p. Fallowed; p. pr. & vb.
   n. Fallowing.] [From Fallow, n.]
   To plow, harrow, and break up, as land, without seeding, for
   the purpose of destroying weeds and insects, and rendering it
   mellow; as, it is profitable to fallow cold, strong, clayey
   land.
   [1913 Webster]

WordNet (r) 3.0 (2006):

fallow
    adj 1: left unplowed and unseeded during a growing season;
           "fallow farmland"
    2: undeveloped but potentially useful; "a fallow gold market"
    n 1: cultivated land that is not seeded for one or more growing
         seasons