Fantasia, meist improvisiertes Musikstück für ein Instrument.
ETYM Italian See Fancy.
A musical composition of a free form usually incorporating several familiar themes.
Or fantasy, phantasy, or fanc; In music, a free-form instrumental composition for keyboard or chamber ensemble, originating in the late Renaissance, and much favored by English composers Dowland, Gibbons, and Byrd.
It implies the free manipulation of musical figures without regard to models of form. Later composers include Telemann, J S Bach, and Mozart.