Source : Webster's Revised Unabridged Dictionary (1913)

Fantasia \Fan*ta"si*a\, n. [It. See {Fancy}.] (Mus.)
   A continuous composition, not divided into what are called
   movements, or governed by the ordinary rules of musical
   design, but in which the author's fancy roves unrestricted by
   set form.

Source : WordNet®

fantasia
     n : a musical composition of a free form usually incorporating
         several familiar themes