The fantasia (also English fantasy, German fantasie, French fantaisie) is a musical composition with its roots in the art of improvisation. Because of this, it seldom approximates the textbook rules of any strict musical form.

In the Baroque and Classical music eras, a fantasia was typically a piece for keyboard instruments with alternating sections of rapid passagework and fugal texture. From the Baroque period, J. S. Bach's Chromatic Fantasia and Fugue, BWV 903, for harpsichord, and Fantasia and Fugue in G minor, BWV 542, for organ, are examples; for an example from the Classical period, see Mozart's Fantasia in D minor, K397 (see Köchel) for fortepiano.

The term also referred in the Baroque era (more specifically British Tudor music) to pieces for viols, characteristically though not always alternating, in this case rapid fugal sections with slower sections in slow notes and sometimes clashing harmonies. Henry Purcell's fantasias are the last Baroque representatives of the breed, although Henry Cobbett, in the opening decades of the 20th century, attempted to resurrect something of this style via a competition, to which works like John Ireland's and Frank Bridge's phantasie-trios, Benjamin Britten's phantasie-quartet (for oboe and strings) and other music owe their existence.

In the Romantic period, two contradictory trends greatly affected the fantasia: one was the decline of formal improvisation as a test of the compositional technique; the other was the move by composers toward freer forms. Chopin's Fantasy in F minor/A flat major, op. 49, combines various keyboard textures of the stile brillante with the classical sonata paradigm, resulting in a work of unorthodox but sophisticated form. Schumann's numerous 'fantasy pieces' are character works on a smaller scale, often bearing descriptive titles.

Further reading

• English Chamber Music by Ernst Hermann Meyer. Reference on the early English fantasy (fantazy, fantasie, fantasia.) Lawrence & Wishart, London, 1946. (Was republished by Da Capo Press, 1971, with ISBN 0306700379.)