noun

• (music, also figurative) A form of instrumental composition with a free structure and improvisational characteristics; specifically, one combining a number of well-known musical pieces.
  "The Fantasia, by Mr. [Pio] Cianchettini, is upon [Gioachino] Rossini's air "Tu che accendi," so often made the theme of piano forte lessons. [...] Mr. Cianchettini's imagination is very vivid and full, and we know of nothing more florid or requiring lighter and more delicate touching than this Fantasia."
• (chiefly art, by extension) Any work which is unstructured or comprises other works of different genres or styles.
  "It is, however, a ceremony of immense antiquity, and the chief civil festival of the year among the Arabs, who love nothing more dearly than a "phantasia" of this sort."