Source : Webster's Revised Unabridged Dictionary (1913)

Tonight \To*night"\, adv. [Prep. to + night]
   1. On this present or coming night.

   2. On the last night past. [Obs.] --Shak.

Tonight \To*night"\, n.
   The present or the coming night; the night after the present
   day.

Source : WordNet®

tonight
     n : the present or immediately coming night
     adv : during the night of the present day; "drop by tonight" [syn:
            {this evening}, {this night}]