Source : Webster's Revised Unabridged Dictionary (1913)

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

   2. The state of being depressed; a sinking.

   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.

   4. Humiliation; abasement, as of pride.

   5. Dejection; despondency; lowness.

            In a great depression of spirit.      --Baker.

   6. Diminution, as of trade, etc.; inactivity; dullness.

   7. (Astron.) The angular distance of a celestial object below
      the horizon.

   8. (Math.) The operation of reducing to a lower degree; --
      said of equations.

   9. (Surg.) A method of operating for cataract; couching. See
      {Couch}, v. t., 8.

   {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.

Source : WordNet®

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