Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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}]