Source : Webster's Revised Unabridged Dictionary (1913)

Sundown \Sun"down`\, n.
   1. The setting of the sun; sunset. ``When sundown skirts the
      moor.'' --Tennyson.

   2. A kind of broad-brimmed sun hat worn by women.

Source : WordNet®

sundown
     n : the time in the evening at which the sun begins to fall
         below the horizon [syn: {sunset}] [ant: {dawn}]