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.
      [1913 Webster]

   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.
      [1913 Webster]

   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.
      [1913 Webster]

            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.
      [1913 Webster]

   2. [Cf. Fallow, n.] Left untilled or unsowed after plowing;
      uncultivated; as, fallow ground.
      [1913 Webster]

   Fallow chat, Fallow finch (Zool.), a small European bird,
      the wheatear (Saxicola [oe]nanthe). See Wheatear.
      [1913 Webster]

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

Moby Thesaurus II by Grady Ward, 1.0:

235 Moby Thesaurus words for "fallow":
   acarpous, achromatic, achromic, anemic, arable land, arid, ashen,
   ashy, at leisure, at liberty, at loose ends, aureate, auric,
   available, backset, barnyard, barren, barton, beige, bled white,
   bloodless, buff, buff-yellow, cadaverous, canary, canary-yellow,
   cattle ranch, celibate, chicken farm, childless, chloranemic,
   citron, citron-yellow, collective farm, colorless,
   cotton plantation, cream, creamy, croft, cultivate, culture, cut,
   dairy farm, dead, deadly pale, deathly pale, delve, demesne,
   demesne farm, desert, desolate, dig, dim, dimmed, dingy,
   discolored, disengaged, drained, dress, dried-up, dry, dry farm,
   dude ranch, dull, ecru, etiolated, exhausted, exsanguinated,
   exsanguine, exsanguineous, factory farm, faded, faint, farm,
   farmery, farmhold, farmland, farmplace, farmstead, farmyard,
   fertilize, flat, flaxen, force, free, fruit farm, fruitless,
   fur farm, gaunt, gelded, ghastly, gilded, gilt, gold, gold-colored,
   golden, grain farm, grange, grassland, gray, hacienda, haggard,
   harrow, hoe, homecroft, homefarm, homestead, hueless, hypochromic,
   idle, impotent, ineffectual, infecund, infertile, issueless,
   jejune, jobless, kibbutz, kolkhoz, lackluster, leached, leaden,
   leisure, leisured, lemon, lemon-yellow, list, livid, location,
   lumpen, lurid, lusterless, luteolous, lutescent, mains, manor farm,
   mat, mealy, menopausal, muddy, mulch, neutral, nonfertile,
   nonproducing, nonproductive, nonprolific, ocherish, ocherous,
   ochery, ochreous, ochroid, ochrous, ochry, off, off duty, off work,
   or, orchard, otiose, out of employ, out of harness, out of work,
   pale, pale as death, pale-faced, pallid, pasture, pasty, pen,
   plantation, plow, poultry farm, primrose, primrose-colored,
   primrose-yellow, prune, rake, ranch, rancheria, rancho, saffron,
   saffron-colored, saffron-yellow, sallow, sand-colored, sandy,
   sheep farm, sickly, sine prole, spade, station, steading, sterile,
   stock farm, straw, straw-colored, sucked dry, tallow-faced,
   teemless, thin, thin out, till, till the soil, toft, toneless,
   truck farm, uncolored, uncultivated, unemployable, unemployed,
   unfertile, unfruitful, unoccupied, unplowed, unproductive,
   unprolific, unsown, untilled, virgin, wan, washed-out, waste,
   wasted, waxen, weak, weed, weed out, whey-faced, white,
   without issue, work, xanthic, xanthous, yellow, yellowish