Source : Webster's Revised Unabridged Dictionary (1913)

Freewill \Free"will`\, a.
   Of or pertaining to free will; voluntary; spontaneous; as, a
   freewill offering.

   {Freewill Baptists}. See under {Baptist}.

Source : WordNet®

freewill
     adj : done of your own accord; "a freewill offering"