The Collaborative International Dictionary of English v.0.48:

Trough \Trough\ (tr[o^]f), n. [OE. trough, trogh, AS. trog,
   troh; akin to D., G., & Icel. trog, Sw. tr[*a]g, Dan. trug;
   probably originally meaning, made of wood, and akin to E.
   tree. [root]63 & 241. See Tree, and cf. Trug.]
   1. A long, hollow vessel, generally for holding water or
      other liquid, especially one formed by excavating a log
      longitudinally on one side; a long tray; also, a wooden
      channel for conveying water, as to a mill wheel.
      [1913 Webster]

   2. Any channel, receptacle, or depression, of a long and
      narrow shape; as, trough between two ridges, etc.
      [1913 Webster]

   3. (Meteor.) The transverse section of a cyclonic area where
      the barometric pressure, neither rising nor falling, has
      reached its lowest point.
      [Webster 1913 Suppl.]

   Trough gutter (Arch.), a rectangular or V-shaped gutter,
      usually hung below the eaves of a house.

   Trough of the sea, the depression between two waves.
      [1913 Webster]

WordNet (r) 3.0 (2006):

trough
    n 1: a narrow depression (as in the earth or between ocean waves
         or in the ocean bed)
    2: a channel along the eaves or on the roof; collects and
       carries away rainwater [syn: gutter, trough]
    3: a concave shape with an open top [syn: bowl, trough]
    4: a treasury for government funds [syn: public treasury,
       trough, till]
    5: a long narrow shallow receptacle
    6: a container (usually in a barn or stable) from which cattle
       or horses feed [syn: manger, trough]

Moby Thesaurus II by Grady Ward, 1.0:

234 Moby Thesaurus words for "trough":
   adit, aerospace, aerosphere, air hole, air pocket, airspace,
   alveolation, alveolus, amplitude, antinode, antrum, aqueduct,
   armpit, basin, billow, bore, bottom, bottom glade, bottoms, bowl,
   breakers, bump, burrow, canal, canalization, canalize, carve,
   cavity, ceiling, chamfer, channel, chisel, chop, choppiness,
   chopping sea, chute, comb, comber, concave, concavity, conduit,
   corrugate, course, crack, crater, crest, crimp, crosswind, crypt,
   cup, cut, dado, dale, de Broglie wave, dell, delve, depression,
   diffraction, dig, dig out, dike, dingle, dip, dirty water, ditch,
   dredge, drill, drive, duct, eagre, eaves trough, egress,
   electromagnetic radiation, electromagnetic wave, empty space,
   engrave, entrance, entrenchment, excavate, exit, favorable wind,
   flute, fog, fold, follicle, fosse, frequency, frequency band,
   frequency spectrum, front, funnel chest, furrow, gap, gash, gill,
   glen, goffer, gouge, gouge out, gravity wave, groove, ground swell,
   grove, grub, guide, guided wave, gully, gutter, ha-ha, head wind,
   heave, heavy sea, heavy swell, high-pressure area, hole, hollow,
   hollow shell, in phase, incise, ingress, interference, intervale,
   ionosphere, jetstream, kennel, lacuna, lift, light,
   longitudinal wave, lop, low-pressure area, lower, lunar rill,
   mechanical wave, mine, moat, node, out of phase, overcast, pass,
   passage, passageway, peak, penstock, pentrough, period,
   periodic wave, pit, pleat, plow, pocket, popple, punch bowl,
   quarry, rabbet, radio wave, ravine, ray, reinforcement, resonance,
   resonance frequency, riffle, rifle, ripple, rise, roll, roller,
   rough water, roughness, rut, sap, scend, scoop, scoop out, score,
   scrabble, scrape, scratch, sea, seismic wave, send, shell,
   shock wave, shoot, shovel, sink, sinus, slit, socket, sound wave,
   soup, space, spade, strath, stratosphere, streak, striate,
   substratosphere, sunk fence, surf, surface wave, surge, swell,
   tail wind, tidal bore, tidal wave, tide wave, transverse wave,
   trench, tropopause, troposphere, troughing, troughway, tsunami,
   tunnel, turbulence, undulation, vale, valley, visibility,
   visibility zero, vug, wadi, water wave, wave, wave equation,
   wave motion, wave number, wavelength, wavelet, way, white horses,
   whitecaps, wrinkle