The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

fantasia
    n 1: a musical composition of a free form usually incorporating
         several familiar themes