The Collaborative International Dictionary of English v.0.48:

Sunset \Sun"set"\, Sunsetting \Sun"set`ting\, n.
   1. The descent of the sun below the horizon; also, the time
      when the sun sets; evening. Also used figuratively.
      [1913 Webster]

            'T is the sunset of life gives me mystical lore.
                                                  --Campbell.
      [1913 Webster]

   2. Hence, the region where the sun sets; the west.
      [1913 Webster]

   Sunset shell (Zool.), a West Indian marine bivalve
      (Tellina radiata) having a smooth shell marked with
      radiating bands of varied colors resembling those seen at
      sunset or before sunrise; -- called also rising sun.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sunset
    adj 1: of a declining industry or technology; "sunset
           industries"
    2: providing for termination; "a program with a sunset
       provision"
    n 1: the time in the evening at which the sun begins to fall
         below the horizon [syn: sunset, sundown] [ant:
         aurora, break of day, break of the day, cockcrow,
         dawn, dawning, daybreak, dayspring, first light,
         morning, sunrise, sunup]
    2: atmospheric phenomena accompanying the daily disappearance of
       the sun
    3: the daily event of the sun sinking below the horizon