The Collaborative International Dictionary of English v.0.48:

Depression \De*pres"sion\, n. [L. depressio: cf. F.
   d['e]pression.]
   1. The act of depressing.
      [1913 Webster]

   2. The state of being depressed; a sinking.
      [1913 Webster]

   3. A falling in of the surface; a sinking below its true
      place; a cavity or hollow; as, roughness consists in
      little protuberances and depressions.
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   4. Humiliation; abasement, as of pride.
      [1913 Webster]

   5. Dejection; despondency; lowness.
      [1913 Webster]

            In a great depression of spirit.      --Baker.
      [1913 Webster]

   6. Diminution, as of trade, etc.; inactivity; dullness.
      [1913 Webster]

   7. (Astron.) The angular distance of a celestial object below
      the horizon.
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   8. (Math.) The operation of reducing to a lower degree; --
      said of equations.
      [1913 Webster]

   9. (Surg.) A method of operating for cataract; couching. See
      Couch, v. t., 8.
      [1913 Webster]

   Angle of depression (Geod.), one which a descending line
      makes with a horizontal plane.

   Depression of the dewpoint (Meteor.), the number of degrees
      that the dew-point is lower than the actual temperature of
      the atmosphere.

   Depression of the pole, its apparent sinking, as the
      spectator goes toward the equator.

   Depression of the visible horizon. (Astron.) Same as Dip
      of the horizon, under Dip.

   Syn: Abasement; reduction; sinking; fall; humiliation;
        dejection; melancholy.
        [1913 Webster]

WordNet (r) 3.0 (2006):

depression
    n 1: a mental state characterized by a pessimistic sense of
         inadequacy and a despondent lack of activity [ant:
         elation]
    2: a long-term economic state characterized by unemployment and
       low prices and low levels of trade and investment [syn:
       depression, slump, economic crisis]
    3: a sunken or depressed geological formation [syn: natural
       depression, depression] [ant: elevation, natural
       elevation]
    4: sad feelings of gloom and inadequacy
    5: a period during the 1930s when there was a worldwide economic
       depression and mass unemployment [syn: Depression, Great
       Depression]
    6: an air mass of lower pressure; often brings precipitation; "a
       low moved in over night bringing sleet and snow" [syn: low,
       depression]
    7: a state of depression and anhedonia so severe as to require
       clinical intervention [syn: depressive disorder, clinical
       depression, depression]
    8: a concavity in a surface produced by pressing; "he left the
       impression of his fingers in the soft mud" [syn:
       depression, impression, imprint]
    9: angular distance below the horizon (especially of a celestial
       object)
    10: pushing down; "depression of the space bar on the
        typewriter"

Moby Thesaurus II by Grady Ward, 1.0:

259 Moby Thesaurus words for "depression":
   Schmerz, Slough of Despond, abatement, abridgment, abstraction,
   abulia, aching heart, agony, agony of mind, alienation,
   alleviation, alveolation, alveolus, anguish, antrum, anxiety,
   anxiety equivalent, anxiety state, apathy, armpit, attenuation,
   bad times, bale, basin, bitterness, blank despondency, blaze,
   bleakness, bleeding heart, blues, boom, bottoming out, bowl,
   broken heart, business cycle, business fluctuations, bust,
   catatonic stupor, cavity, cheerlessness, cleft, comfortlessness,
   compulsion, concave, concavity, contraction, cooling off,
   couchancy, crater, crena, crisis, crushing, crypt, cup, cut,
   dampening, damping, death wish, debasement, decrease, decrement,
   decrescence, deduction, deepening, deflation, dejectedness,
   dejection, dent, depreciation, depth of misery, desolation,
   despair, despondency, despondentness, detachment, digging,
   diminishment, diminution, dimple, dip, discomfort, discouragement,
   disheartenment, dismalness, dispiritedness, distress,
   distressfulness, down trip, downcastness, downer, downheartedness,
   downturn, dreariness, drilling, drooping spirits, dying, dying off,
   economic cycle, economic expansion, economic growth,
   economic stagnation, elation, emotionalism, euphoria, evil day,
   excavation, expanding economy, expansion, extenuation, extremity,
   fade-out, fold, folie du doute, follicle, funnel chest, gash,
   gloom, gloominess, glumness, grief, grievousness, growth, hack,
   hard times, heartache, heartlessness, heavy heart, heavy weather,
   high growth rate, hole, hollow, hollow shell, hollowness,
   hopelessness, hypochondria, hysteria, hysterics, impression,
   incision, incurvation, incurvature, incurvity, indentation,
   indifference, infelicity, insensibility, jag, jog, joggle,
   joylessness, kerf, lacuna, lamentability, lamentation,
   languishment, lessening, lethargy, letup, low, low spirits,
   lowering, lowness, lowness of spirit, lying, lying down, malaise,
   mania, market expansion, melancholia, melancholy, mental distress,
   miniaturization, mining, misery, mitigation, mournfulness, nick,
   nock, notch, obsession, oppression, pain, painfulness,
   pathological indecisiveness, pathos, peak, peaking, pessimism, pit,
   pitiability, pitiableness, pitifulness, pocket, poignancy,
   preoccupation, probing, proneness, prosperity, prostration,
   psychalgia, psychomotor disturbance, punch bowl, rainy day, recess,
   recession, reclining, recovery, recumbency, reduction,
   regrettableness, relaxation, sad times, sadness, sagging,
   scaling down, scoop, score, scotch, self-destructive urge,
   sharpness, shell, shortness, simplicity, sink, sinkage, sinking,
   sinking heart, sinus, slowdown, slump, socket, sorrowfulness,
   spiritlessness, squatness, squattiness, stormy weather, stumpiness,
   stupor, subjacency, subtraction, suicidal despair, supineness,
   taedium vitae, the blues, tic, trough, tunneling, twitching,
   unhappiness, unresponsiveness, upturn, vug, weakening,
   weariness of life, withdrawal, woe, woebegoneness, woefulness,
   wretchedness