Source : Webster's Revised Unabridged Dictionary (1913)

Nightfall \Night"fall`\, n.
   The close of the day. --Swift.

Source : WordNet®

nightfall
     n : the time of day immediately following sunset; "he loved the
         twilight"; "they finished before the fall of night" [syn:
          {twilight}, {dusk}, {gloaming}, {evenfall}, {fall}, {crepuscule},
          {crepuscle}]