capriccio

from WordNet (r) 3.0 (2006)

capriccio
    n 1: an instrumental composition that doesn't adhere to rules
         for any specific musical form and is played with
         improvisation

from The Collaborative International Dictionary of English v.0.48

Capriccio \Ca*pric"cio\ (k[.a]*pr[=e]t"ch[-o]), n. [It. See
   {Caprice}.]
   1. (Mus.) A piece in a free form, with frequent digressions
      from the theme; a fantasia; -- often called {caprice}.
      [1913 Webster]

   2. A caprice; a freak; a fancy. --Shak.
      [1913 Webster]