Definition: nighttime

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Source: WordNet (r) 1.7 

nighttime
     adj : happening during or appropriate to the night; "nighttime
           attire"; "the cat's nighttime prowls" [ant: daytime]
     n : the time after sunset and before sunrise while it is dark
         outside [syn: night, dark] [ant: day]

Source: Webster's Revised Unabridged Dictionary (1913) 

Nighttime \Night"time`\, n.
   The time from dusk to dawn; -- opposed to daytime.